Following review WILL contain SPOILERS.
Let me preface this review by stating, that what I'll write here - I'll write with the best intention in mind, letting know potential DMs what to prepare for and providing constructive criticism to the author. What I'll write here is rooted in my belief that DMs resource to pre-made modules for three main reasons:
1) to run an adventure when they have no time for prep-work or writing their own material
2) to canibalize maps and/or encounters
3) to get some inspiration for story-wise when they feel burnt out
...and I believe this adventure falls flat when it comes to all of the above.
I will start with biggest issues and work my way down to nitpicking.
Adventure modules are not fiction (in a literature genre sense) but rather utilitarian texts that provide instructions for a DM (beeing primary target audience) to help him execute a game session. Therefore it should not surprise the reader, nor it should contain plot twists, providing all relevant and ctitical information in advance. Don't get me wrong - adventure as presented at the table can (sometimes should) have plot-twists, but those should surprise players, not the DM. In example - if ten chapters into the adventure it turns out that main villain is in fact a woman, and one of the PC's mother at that - DM should be informed of that fact at the very beginning. That is why module should not be written like a movie script - following PCs, assuming their actions in precise order for it to work. And that is how "Trader's Pass" is organized. It gets "railroad-ey" which is RPG sin in its own, but most of all - it's not providing DM with much needed guidance.
Some examples: in the very first chapter PCs are asked to deliver two letters. Contents of those letters are not disclosed, nor they are ever mentioned again. And the recipent is found dead in the end of the chapter. What if PCs choose to open those letters? I know letters were just a MacGuffin, but players don't? What if they think higher of the adventure author and come to a conclusion "oh, this is one of those intrigue-investigation type of adventures, I wonder what was so important that it got the merchant killed"? Similarly the Wand - it is mentioned in the first chapter unbeknownst to players, then mentioned again on page 35, then it is a subject of a "delivery quest" (pg 39), and then... disappears. Even though module final is centered around meeting Adel, the wand is not brought up, neither DM is informed what it does. NPCs might decieve party, NPCs might say to PCs it's "need-to-know" basis. Well... DM needs to know. Preverably at the very first time it is mentioned. Given what levels party reaches there come skills and spells into play - they might want to attune to it, they might want to identify it or even use it blindly. If not the specific write-up, then at least details on what kind of power or magic it is affiliated with. There are many instances of inconsistencies and omissions like those above.
Another concern is, that I feel like the module was never properly tested. It makes a lot of assumptions on party behavior, that on the first glance are illogical. Have the author ever met players? Those inquisitive active creatures that stick their noses everywhere? Letter being one example, but there are more. Players are given delivery quest with a feeling of urgency (the merchant will cross the mountains soon) and yet the adventure assumes they will stay for a night in the inn instead of pressing on through the night. They are tasked with finding out who is a grave-robber, yet cemmentary stalking yeilds no effects - instead they are assumed to sit around in bars, drinking, gossiping around about wine prices. Adventure assumes they will case the house and learn critical information before confronting the farmhouse necromancer head-on. That was actually the railroad choke-point that killed my party.
Story is lacking and uninspired. Calling it a "campagin" is a little generous. Those are mini-adventures that could (with a lot of leeway) be considered loosely connected, but there is no overarching story arc. There is an orc invasion in the first part (though mini-adventures themselves don't seem to be connected that well), but the second part with the whole journey through the mountains and City of Narion seems to be crudely glued to it. I also feel the invasion would have a bigger emotional impact on the players if Alor Valley would be more fleshed-out, and PCs had a chance to bond with NPCs.
Encounters are not balanced, not interesting nor thematic. They just seem to be thrown together as it was convinient for the author. I also don't like the whole "XP doesn't match up, so just, uh... give PCs some levels and stuff". It seems like the author wasn't willing to commit enough effort to do the math. As far as I understand milestones are a valid advancement technique in 5e - it just seems lazy. Given that there are no battle maps - all of the above disqualify encounters as an asset to canibalize for one's own adventure. The same goes for maps - those doesn't seem to stick to one style in terms of how clear and readable they are. Maps in later chapters are dark blurry mess completely unintelligible. It would work better if those were hand-made maps drawn with a black marker on the graph paper - similar to the inn map on pg. 14.
There were some nice parts in "Trader's Pass". I liked the whole "big battle mission". In the face of overwhelming forces of the enemy PCs are given a simple job - "We will distract them, and take them head-on, you destroy the thingy". That makes the battle somewhat a climactic backdrop not bogging down a session with mass combat simulation.
I also liked the idea behind the family feud, though have some issues with previously mentioned "assuming PCs will do X" stuff. I also feel this particular part should be given more background and could use more fleshing out. At this point Calimars and Farins are just numbers.
I like the idea behind scheming orc shamans that prepared Valley for the invasion, tried to set up enemies within and weaken defences but I feel the whole plotline got underutilized.
Summaries of the chapters and missions are a nice touch, though insufficient when the whole thing is written like a path to be followed.
I know that all of above criticism might feel a bit harsh, though I believe the campagin have a little bit of potential and could be much better if the author would be willing to put some work into it. I'd like to offer some suggestions, feel free to ignore them or disagree:
1) I think it needs a thorough rewrite. Writing the whole module from the perspective of the PCs is pointless. It needs a recognizable villain presented at the very beginning, his plan and the timeline for the whole Valley from his perspective. "If players would not intervine he plans to do X and Y". Also "Y days/weeks ago shaman contacted a death cleric and started supplying bandits with weapons". That way DM has a clear understaning of what is going on and what is going to happen and can react accordingly when surprised by unorthodosx player approach. Some motivation for the invasion should be also given beyond "we are orcs, we be bad, arrrr" - DM can plan ahead if he knows orcs are driven here because they are banished faction, or their homeland was plagued with famine and they need land do settle.
2) Create a compelling villain and a mid-level boss as his lieutenant. Orcish shaman would be great - name him, specifically tell players he was the one that provided Lamdin with the altar and thugs with weapons, perhaps let him slip away from PCs grasp - killing him would be so much more satysfying.
3) Alor Valley should be fleshed out more. We get some geo-economy info at the beginning, couple of locations and NPCs, but for the final battle to have impact - PCs should be rooted in the Valley. Adventure could use NPCs summary at the very beginning (not only shopkeepers, but also important NPCs that appear later) so they could be incorporated into character backstories or bonds. The Valley seems empty - perhaps allow some exploration, maybe incorporate sandbox, detail key locations and couple "places of interest", some random encounters (not only combat ones, but also thematic like sick people from Meel, sailors on the raft or farmers delivering their goods to the city).
4) Give DM tools for improvisation and bringing Valley to life. Foreshadow. Random tables could change over time reflecting changes and PCs inpact on the valley or foreshadowing events to come. Good example would be Lamdin - players should be familiar with rumors about him way earlier before even Sardol informs them about the corpse-snatching problem. That way they can connect the dots earlier instead of making illogical decision "we have a quest but let's go to bar and discuss wine gossips"
5) Fix the MacGuffins. If there are letters, at least let us know what is inside. If there is a wand - what it is about. If there is a Sardol's kid - let PCs meet him earlier, let them get to know him (perhaps talk to him while waiting to enter Sardol's office before the first quest). Perhaps Sardol's wife is a childhood friend of one of the PCs?
6) DO NOT assume player actions. Present the DM with problem and circumstances - not the solution.
7) Better connect the last part to the first part. It's tricky as they are so distinct. Perhaps let players find clues to the location of underground pass during their fight with the orcs? Maybe the pass needs de-orc-ization? Maybe point out at the beginning so the DM would remember to make Orion regularely elude to Adel, so their trip would not be so out of the blue?
8) Flesh out the City of Narion. Seriously - this part of the adventure needs more than 5 pages. Given that it is at least twice the size of the Town of Trader's Pass it needs points of interest, key NPCs, locations, flehsed out town adventures, encounters, plot hooks, disctricts, perhaps some faction dynamics. It could be stretched for 4-5 game nights worth of content. Given what is summarized in the background and what is actual content of this part it seems to be very underdeveloped. Take into account that 5th level is the time, when PCs get involved in local politics. The struggle for power between two brothers could be an interesting story arc, but PCs need to have a way to learn about their agendas. Narion itself could be a story arc at least as long as the whole Alor Valley invasion plot-line. Develop it or cut it out entirely making it a second mini-campagin as a sequel.
9) Consider working with an artist who could make better maps - both for the cities and adventure locations.