I was following these instructions. I essentially followed through that list, changed the widescreen mod's resolution, then added the tweaks pack later on. I can't actually view that page right now for various reasons (although I'll be able to in a few hours), so I'm not sure if I put the tweaks pack in too late. If I did, would this be the cause of the issues?
has already answered this question but Yes this will cause you issues every time… mod order is an exact science and widescreen MUST always be last.
As I say, until I installed the tweaks, everything was working fine and I'd been playing another successful game. Are there any specific tweaks which are likely to have caused issues (other than the beta ones, which I didn't install)?
Yes there are issues with every mod and now days the good mods all list their issues on the webpage or in the readme.
Due to lack of spare memory at the time, I didn't make a backup folder, but as it happens I have the games on disk (possibly from before GOG started, in fact), so reinstalling the game shouldn't be too hard if I need to.
Gog game = don't patch but the old disc version = do patch.
Will it be safe to import my character from the corrupt game once I'm done reinstalling, or should I cheat the stats to his levels and recreate him from scratch?
Yes you can safely import characters as long as the save file they came from will load without crashing the game… even if the dialog talk in that save game is corrupt.