I have just checked it out following your instructions.
The crash occured immediatly right after the succubus started resurrecting those guys. Yeah, that's pretty inconvenient to have this stuff emerging in stable builds.

Concerning "impulse12" related crash. You gotta have super shotgun and quad super shotgun in your possession.
Try to scroll previous/last/next weapon with any button you assigned to execute that action. The game freezes then. 

1) When I press "load game" then load the savegame and then press ESC button it shows me "load game" menu instead of Main Menu of Quake. That's a bit inconvinient because I usually accidentally press "Enter" button and load a save game while forgetting to save the game after updating my game progress thus it results in loading an old savegame :D
Is there an option to disable this kind of a "Remember last menu" feature ?

2) If you run across a nailgun ammo box while firing the nailgun - it stops firing for a half a second. Really strange. Same applies for Supernailgun. 

I think the impulse12 in Ne_ruins would need to be fixed within the mod. It wasn't made with impulse12 support and since that quad shotgun is an extra weapon, it causes problems. I believe s.o. actually made a patch for that, but I could be wrong. The succubus resurrection crash is definitely to blame on Mark V, though.

I also cannot run Ne_Ruins without assigning more memory to it via -heapsize 512000. Otherwise I get a crash right after leaving the intro map (Hunk_Alloc). 

At the introdution map of Quake, if you don't move your mouse and you go forward (move forward command) the player starts to look down at ladder a bit and if you go back (move back command) to the top the player centers his view back. If you move your mouse at the beginning - this behavior disappears.

Strange stuff, I think there must be a console command to disable such a behavior, but I can't find one. 

When I go to multiplayer section of Dissolution Of Eternity (or any other mission pack) I don't see any maps of this expansion available for deathmatch or cooperative gameplay. There are only original Quake SP and DM maps displayed. Same for Scourge Of Armagon. Is there any way to make the maps displayed in multiplayer? Because it's impossible to play mission packs in coop or dm modes.

Is there any specific command line to enable creation of directories called "Save" and "Screenshots" inside a corresponding mod folder or main ID1 directory? It would be nice to have Mark V storing savegames and screenshots inside these particular folders. 

You shouldn't remove this command since it is actually used. For example I change my refresh rate between my laptop and stationary PC. On laptop I'm running 60 hz and 100 hz on PC. So I use this - "vid_refreshrate" to set it up. 

It would be nice if Mark V had more than 20 save slots for savegames. 50-100 would be great.
Should be implemented :)