I really don't have that much invested in the corrupted save, maybe a weekend's worth of work but my inner geek would really like to sort this out for any future incidents. Learning raws and graphics along the way sounds pretty fun.
I understand. No worries.
To answer the last question there, it happened at first with all graphics packs aside from the ASCII. Then for whatever reason (likely the culprit) even that stopped working, same error I used to get trying different graphics packs with that save. Two or three times I've tried, other saves working fine, then one day I went back to ASCII to play the newer fort and got the error.
You lost me here. Although, maybe this is related to LNP. Hold on . . .
Okay, this is what I believe happened. Up front, I have never used LNP and I have very limited knowledge about the LNP. I've only briefed myself back in December from the thread I linked to earlier when these issues came up. In short,
the LNP is the problem in your case.
My guess is that you are not manually switching your graphic sets and that you used the LNP interface to switch out to different sets and even ASCII, correct? In doing so you inadvertently edited the object raws of your save. In other words, your attempt to fix the problem using the LNP made the situation worse, unfortunately.
The actual problem is that years ago someone (IIRC Phoebus) invented "tileset magic", where via a number of colour and transparency tricks a carefully-drawn tile could look like many different things in-game. This required changes to the object raws, and those are what cause all the compatibility trouble - most graphics packs are an unholy combination of tileset (hence data folder), mod (hence raw/objects), creature graphics, and now TwbT/DFHack files too.
Based on my limited understanding of how the LNP switches graphics (maybe someone will confirm or deny this) you are editing (or more specifically switching) object raws every time you use the LNP just to change graphic sets or turn them off. That in itself is . . . messy.
To rescue your save, you need
the original save file before you tried switching graphic sets in the LNP. If you have that, you'll need to manually transfer the save to the version you want rather than rely on any method the LNP provides. Also,
avoid using the LNP all together with this save.
If you need any auxiliary programs that come with the LNP in order to play, just download the ones you need separately.
I hope that helps.