AC questions for the experts

EPA 609 is pretty easy to get. I have 608 Universal and don't mess with auto AC outside of my own cars. You can download study material and schedule testing through www.escogroup.org.

You might find some useful info at this link. Its a corvette R134A retrofit but the same principles ought to apply:
Updating Air Conditioning from R12 to R134a – The Corvette Restoration Page

I think the R134A won't be compatible with the seals in your R12 system, so you'll probably see leaks. R134A is still pretty cheap so it'll get you by until you can do a proper conversion. As far as using R22, it operates at different pressures. I wouldn't try it myself. In commercial and industrial work R22 and R12 are not considered equivalent and one is never substituted for the other (even after a system rebuild) because the operating pressures are so different.