From the limited amount of screenshots, I can say that the amount of details being presented is "acceptable", although more will definitely improve the over all visual feel. The detail style being used is very 90s-ish, and in this day and era, people generally expect more eye-candy from doom maps. Adding some computer pannels and flickering lights as well as some damaged walls will liven things up a bit. Also, the details between the maps seems rather inconsistent in quantity. The 3rd screenshots shows much more detail, using SUPPORT2 borders and lights on the ceilings, where as the first 2 don't have any of that. The second secreenshot is especially blan with almost no details to be found other than the ones that are graphically placed on the textures. I recommend some crates, barrles, lights (sprite lamps), or anything to be added. It simply looks unfinished. The out areas should also have more or a "yes this is a building" feel to them, instead of just a blank wall. However, the detail style is consistent with classic doom, which might be your overall goal.
As for gameplay, I can say nothing from screenshots. I haven't played it (i'll try it if I have the time), so I wont make any half-assed assumptions. The map-layout is that of a circular-flow style, which generally makes the game more enjoyable, imo, than lineage or back-and-forth maps. Although I cant be sure. Still, from the amount of red dots compared to the amount of brown and yellow dots, i can ay that the map seems a bit tight on the ammo (unless all those red dots are mosnters that drop crap). I also see next to no health items on the map, which might be a pain with that many monsters. It's not usually wise to only put as much ammo as the map-maker needs to beat the map, since he knows were all the monsters and items are. The monster placement also seem cluttered. There are many areas where it's filled with monsters. Unless you want to accomplish a "Doom-Gods Only" style of gameplay, I'd say there are just too many monsters for some of the areas. But again, I cant be certain.
As for legal issues, nothing will happen even if this mod was illegal. I've seen doom wads where it was just the normal doom maps but that's copied and pasted together and the player uses a;; the heretic weapons.
EDIT: having played the demo, I've changed some views on this.
The maps have a good amount of detail, even though it looks 90s-ish *which is fine). The maps are pretty cramped and doesnt have the open feel like the normal doom maps (which is also fine, which ever suits your style). There are areas that needs detailing, such as the computer with the COMPSPAN textures. Map02 also has an outdoor area near the begining where there is a Nukage pool. I can step out of the window and go into this area, but I cant get back in (shown in your second screenshot). The amount of monsters to weapons is a problem, especially the type of monsters. The first map was played with pistol almost all the way through, which can get a bit annoying. The second was better once I the player has a shotgun and chaigun. still, there is an uneven amount of mosnters within the maps. Some areas are completely filled (usually the hallways), while others (generally the larger rooms) are next to empty. The most annoying part of map02 is the back-and-forth game-play. It was running to a switch, hitting it, then running back to a door that you know was gonna open. It was like that almost through out the entire map. The machingun is rather useless, since there is a chaingun. The sounds and sprites for that weapon also needs a lot of work.
generally, pretty decent set of maps. And game play was managable.