[Facial Piercing Mod]

I know necroing is frowned upon but did you ever find anything relating to piercings or did you just give up and create your own.

[Randomly walk through certain objects]

Likely a navmesh problem, but if you're bothered by it and are certain it isn't a navmesh problem left click the object and type 'tcl to turn the collision on and off.

["Falling through floor" in Nipton Town Hall]

Are you using any mods which alter Nipton from it's vanilla state? if so this might very well be the case; do you have the latest version of CASM, NVAC; FNVSR or New Vegas? try installing an unofficial patch such as Mission Mojave this fixes over 27,000 bugs, glitches, errors etc...

[Game Suddenly Unplayable]

Have you tried wiping the game from your HDD (or SSD) via Steam; *.ini configuration files (generally found in C:\Users\(Profile Name here)\My Documents\My Games\FalloutNV\) an all, reinnstall it and and play the game using graphics options one lower than what you previously used. If the problem persists it may be the amount of free space on your Mechanical Disk (Try Defragging it, this should help with performance a little). try using this to keep your mods seperate from the game so uninstallation is easy if a problem occurs with the game, If you still have this problem it may be your saves; use this as it might help with your problem and greatly reduce the file size.