Ok, I've already got the insider account and app.
1. How do I un-enroll from the dev preview?
2. Are there any bugs with 8.1.2? What benefits does it have and what could I possibly lose?
3. If I unenroll from the dev review will I be able to get it back? I ask because my 925 hasn't gotten Denim here in France yet (I don't think) and I don't want to lose what I've got now, even if it's not working great. Any way to check what insider will offer me first without going through the unenroll process?
1. Open the PFD app, and uncheck the box to unregister from it, and tap "done" to unenroll from it.
2. You won't lose anything (if you have the unlocked model, and aren't enjoying some carrier-specific thing). The benefits aren't huge (besides the sorted settings menu, and other minor things), but the app load times have improved noticeably, as I said. It might be worth your while to try it out, maybe it will fix the crashing bugs.
3. In all fairness, the friend of mine who got the 8.1.2 update on his 925 was already running the Denim update, and AFTER that had the latest PFD updates installed. His phone was the Bouygues carrier model (unlocked) which didn't have Denim release, but he flashed the international French country variant ROM and it works just fine; any ROM for the RM-892 will run on it, so if you match the RM model code of your device you can get the official Denim ROM of another country and it'll run fine. You can do that using the process listed
here. It's not as complicated as it looks. Your phone will not downgrade to an older version if you unenroll from the PFD app. The only way to check if you're getting the 8.1.2 update through Insider is to try it for yourself. But yours and my friend's case are similar enough that I am pretty certain you'll get it if you do what he did.