Stabbey

Do you have the "Auto-finish drawing" option selected in the Menu Bar? Un-select that, and manually draw the portions of the sector. That usually keeps the editor from guessing incorrectly about what portions of the void you want to be sectors.

If you put items visible in a location, the player usually expects to be able to get there. They'll think there is some secret way to get in, and they'll spend a bunch of time searching for a secret which doesn't exist. Secondly, yes, put a texture on the back side. It's good practice to do so. You don't want to get into sloppy habits as a mapper.

What msx2plus said. What you want is not possible in classic doom2 format, outside of extremely specific pre-programmed cases, like all Barons dead in E1M8, all Spider Masterminds dead in E4M8, all Mancubi dead in MAP07, or all Commander Keen Dolls dead in any other Doom 2 map slot. Other than that, your option is to use a format which supports scripting.

Keep in mind though, that a lot of players hate that.

You can open the standard WAD files just like any other WAD. I strongly suggest that you make a backup of the original WAD first, though. I'll also note that you shouldn't release your changed WAD to the public, because the original work isn't yours.

I think it's only possible with a port which supports scripting, so no to Boom, and yes to code.

You can probably find some at Realm666 (Remember to respect the original author's permissions and give credit). Creating your own will either require making new textures, new sprites, or 3D models. The textures are the easiest things to do, but will look the least real.

Total conversion projects are a LOT of work. It's slightly easier to do just sprites for enemies, but even so that's still a lot of work. You spent most of your OP talking about the lore and idea, which I didn't bother reading, because that won't help with actually completing the work. Get Slade 3, open up one of the doom wads, and find the sprites. The basic pink Demon enemy has 57 sprites, and it only has the one attack. An Arch-Vile has 144 different sprites. It's so high because each enemy is going to need eight different sprites per frame of animation (one for each angle of rotation). That handful of crudely drawn blobs in the sketch isn't enough reference to make sprites for all those angles of rotation. At the very least to start with, you need to know what your enemy will look like from the front, the back, and both sides, and it needs to be a clearer and more defined image than the provided drawings. Start with drawing those profiles and refining them before you think about starting sprites. EDIT: Also, questions from someone wondering how to do a thing go into either the Doom Editing or Editing Questions sub-forum. The Tutorials forum is generally people who already know how to do a thing explaining how to do that thing.

If you want help debugging a map, it would probably help to attach the map, not just the crash report.

I'm still around, technically. But that's all I have to report.

Weapons are sprites, not textures, so you want to be looking for tutorials on how to replace weapon sprites. New sprites need to go between SS_START and SS_END markers in your WAD, which you create in Slade 3.

It's usually just a cheap shot by authors to exploit human reaction times. Ha-ha, you're taking damage because a trap you had no way to see dumped monsters point-blank in your face, aren't I a smart and clever mapper? I did use something like that myself in "More Tricks and Traps than you Require" to...

*sigh* From the provided screenshot, Am I correct in assuming that the line(s) which trigger sector 1 are the green lines on either side of the Tag 1 sector? If so, that might be your issue. The little 'tick' on those lines is pointing into the regular floor. I suspect that SMG Man is correct: the type of trigger you're using takes the texture and effect from the sector the ticks are pointing into - which, of course doesn't have the lava texture or damage effect.

That's not recommended, you should only import textures you're actually using. There's no need to inflate the size of your WAD with hundreds of textures the map isn't even using.

I think that you need to have the linedef you walk over be pointing into a sector with the correct texture and sector effect. The trigger will take the texture and effect from the sector the line if facing into, not the texture and effect of the actual lower sector. You can do this by making a very thin sector in front of the trigger with the appropriate texture and effect, but leave the wall textures of that sector empty so the floor gets "painted" over the crack to hide it. The height of the thin sector doesn't matter, just the texture and effect, so you can have it shallow enough (-24 below the regular floor or less) and monsters will still walk over it fine.