I finally got it to work by creating a new folder and launching it via drag and drop from there, and it worked fine. Having gotten it running and finishing the demo, I am going to be very critical in this review. Please don't let my words dissuade you from continuing to develop this project- you have a fantastic start here, and as I've stated on multiple occasions, the first Lost Civ is a masterpiece of modern Wadmaking, so I want to see the sequel become the best it can possibly be. So I will try to be detailed in the hope of offering things to consider as you go forward with your development.
Visuals- There are definitely a few moments, such as first disembarking from the boat and when you approach and autosave point, for instance, when the game overwhelms the engine and the framerate tanks, but its not gamebreaking. In that regard, much like the original, it is an absolute feast for the eyes. Your work on small detailing is impeccable and creates a believable environment that is begging to be explored inch by inch. I love the different interiors from the bars to shop windows to the balconies of the hotel, and that genuine feeling of a world of luxury fallen into decay. This is above and beyond your strength, and I have no suggestions to improve. Keep up the great work.
Layout- The first game was heavily into exploration, but it takes on a new depth here, thanks to using items in the inventory to progress and the light puzzle elements that this entails. Simply by exploring the environment, I was able to find all the components I needed to solve a puzzle, and sections like the generator in the garage or fixing electrical wiring all seemed intuitive; this wasn't Resident Evil or Silent Hill, and it gave me satisfaction because it made sense.
On the flipside of that, the navigation of the environment can be confusing at times, and while this was occasionally a minor point in the original, here finding my way around proved confusing at first. I wasn't sure what to do in the garage, and I was wandering the alleyways at the start looking for the next area for several minutes. I think perhaps using monster or item placement could make this a bit clearer. Again, not a deal breaker, but some players might find being lost in this civilization more frustrating than others. The only time I was about ready to quit due to confusion was the big fight near the construction site and hotel before the beach near the end of the map; I think your use of crates can be confusing since the environments are so detailed and blocked paths are easy to miss.
Gameplay-I'm going to separate gameplay from combat since I have much more to say about the latter, and I'll use this section to touch on the weapons and navigation. I normally hate platforming in Doom Wads- too often it feels forced and clunky, but thanks to your integration of manteling and very simple jumping sections, it never felt frustrating to move around here. The guns are- a mixed bag. I felt woefully underpowered at the start of the map and the initial single shotgun handles atrociously, its accuracy almost completely nullified if you aren't close to point-blank when firing. The slow firerate of the plasma blaster pistol feels terrible when faced down with multiple chaingunners (especially the Uber variants) and the auto-pistol just feels flimsy. The axe is a joke of a weapon in this current build- too slow and weak to be effective in combat, and even a chore to use breaking those surprisingly resilient crates (I think those big ones might have a little too much health, as I whacked away a good six or seven times before they finally broke). In brief, I think the starting weapons could use a little more heft, because I often felt unfairly outmatched with the inaccuracy and lower-power of the weapons combined with the low starting health. Ammo and health themselves were a huge problem in the first half of the map until I got the riot shotgun (the one from Winter's Fury), which improved the combat dramatically, but I was dying a lot and became extremely frustrated by the slow trickle of supplies at the start. I get that it's an uphill battle, but it isn't fun and feels like it could use a lot of rebalancing. Maybe medkits could be a little more effective or enemies could drop more ammo when they die? Sometimes I got a few shells or a clip or two, but this was never consistent.
Combat- This brings me to the final section, which I currently feel is the weakest part of the Wad- you've nailed the exploration, the design is excellent (with maybe an additional marker or two so the player doesn't get too confused), but the combat itself ranged from decent to outright infuriating; I was never having a good time. I played on HMP, and I got hurt a little more than just Plenty. I'll be blunt- there were many, many times this Wad left me angry and frustrated, as projectiles came seemingly out of nowhere, I was sniped from all sides, my health dropped from 50 to 0 in a matter of seconds, and I was cursing in the search for better guns, ammo, and more health. The sniper section epitomizes my frustration- the enemies kill you in seconds before you have time to see where the projectiles are coming from. I don't think the custom enemies are the issue here, necessarily- its that its too much, too quickly. From the homing attacks of the new imps to the instant-kill psychic wave of the Arachnorb, to the poison projectiles of the Roach and flying Mancubus, and the hyper-fast stream of projectiles from the former human (the one with the yellow plasma bolts) I just didn't like this very much at all. But being surrounded and fighting off multiple powerful enemies is part of Doom; look at slaughtermaps after all, and those don't necessarily have to feel unfair of unbalanced despite the extremity of the fights being part of their design philosophy, right? Hurt and Bad Religion both use a lot of custom monsters, and those are great maps. So... why isn't it working here? Part of what I think makes things frustrating is how easy it is for enemies to hide in the detailed environments and ambush the player- this was one of the few big problems I had with the first one as well, but since they were standard Doom enemies, once you knew how to spot them, you could get the upper hand because the balance still felt right. With such a complex environment seeping with so much detail, and the homing and high-damage nature of so many of the new enemies' attacks, coupled with the health and ammo restriction that forces constant scavenging, things just never tilted the other way for me, and I spent the entire map annoyed as I was dying and savescumming just to move a few inches. I'm not an expert in this field, but I think rebalancing the weapons and monster damage and attacks could be helpful, and pay special attention to how often RNG puts the player at a disadvantage; my shotgun shells were always dwindling with the chaingunners and Mancubi; the sniper rifle's damage output seems all over the place- RNG might not be the best approach with a weapon like that, or maybe just tighten the range?
On a final note, the NPC dialogue might benefit from a dedicated translator or translation app or service. I'm not trying to make fun of you here; I've worked with students in my last job who barely spoke any English and using any language different from your own is very difficult, as I found when trying to speak Spanish to them in return. This again wasn't gamebreaking- I could understand what was being communicated over 90% of the time. It's something to consider addressing, if you have the time, just to create a more polished final project.
To summarize in brief- keep building on what you have here, but maybe consider tightening up the combat, removing a few monsters perhaps or buffing some of the weapons for accuracy or damage or reworking some of the custom monsters' behaviors and attacks. Take some time to reconsider the enemy placement and how health and ammo are distributed, particularly in the first half of the map. Or don't. It's your Wad of course, and at the end of the day, playtesters like me can only offer suggestions, in the hopes that the final project will be even better for it. Thank you for releasing this demo, and I very much look forward to playing more of the game as it develops.
Best of luck and success,