Probably not a great idea, since that joint does not have a lot of play like an outside corner does (e.g. you can't adjust the metal corner very much in an inside corner). Just use pre-creased paper tape for the corners. Watch some videos online for technique.
I wouldn't try it. They are pre-bent at a 90 degree angle and I thing bending them to fit your 22.5 would be difficult.
Yes, it worked quite well for me.
No nails, just compound. Sounds like perhaps your house is settling and that is causing the corners to "come apart" just slightly? I suppose you could add this atop the existing drywall/corner tape. You'd have to "float" the compound out a bit farther from the corner so as to make the transition smooth and seamless.