There's a reason it's recommended you completely disassemble a revolver, clean & then reassemble before your first trip to the range - and you just discovered why.
I read elsewhere about making a wooden jig for the cylinder, putting your nipple wrench into the chuck of a drill press, then turning it by hand (NO power) to remove stubborn nipples. The extra downward pressure exerted will prevent the wrench from slipping & stripping out. The previous owner of my new-to-me ROA installed the nipples with red Loctite & knowledge about this little bit of extra leverage would have been handy at the time.
A little internal wipedown never hurts & gives you opportunity to inspect all internal parts for rough surfaces. Occasionally, Piettas escape final inspection with a little nib left behind on the inside radius of the cylinder bolt - it causes erratic cylinder movement & lock-up, and is easily remedied with a little gentle Dremel work.
As for screwdrivers, I got a set from Harbor Freight with a 20% discount coupon. Nevermind the fact they didn't fit very well, that's where Knowledge of hollow-ground screwdriver blades can be applied to some precision Dremel fitting. I now have a set that is made to precisely fit my Colts and Remingtons. All for under $10 + some gentle hand shaping.
"Hey Look! We've got Guns ... and We've got Snacks!"
- Cdr. Samuel "Sam" Axe, USN, (ret) -