If your multiplayer create/load/join game is crashing even though you've setup for multiplayer properly, and still want to use widescreen mod at your full resolution (as widescreen mod cannot be set into windowed mode at your full screen resolution), make sure you have EAX disabled in the game, along with any EAX software acceleration (such as Creative Alchemy). I tracked it down to it, and now I can play multiplayer at my full screen resolution with the widescreen mod, instead of going through the hassle of starting the game windowed at lower resolution.
Same thing goes for the Character Import crash (be it single player or multiplayer). I can't for the life of mine understand how having EAX in the game can affect the character import process, but sure enough, game crashes every time I import a character with EAX. Works flawlessly without.
If you're experiencing crashes in multiplayer after loading the game, make sure the host controls all NPC characters you might have (tested). Also, having two players control different NPCs causes massive lag for the other (non-host) player, and causes extreme increase in loading times for both players.
Also, if you're experiencing graphical corruption in the video test while changing your config, make sure you have all overlays from third party programs disabled (mumble, MSI Afterburner etc...), these can also cause flickering in game.
Lesson to be learned - third party applications can influence infinity engine games in very weird ways (for example, BG1 installer not working if you have Chrome running). If you can't find a fix for a weird technical problem, make sure it's not caused by some other program.