The Nitrogen gives lots of extra pressure to help find the leak. It would be legal to fill with R22 AS LONG AS YOU RECOVERED ALL OF IT AFTERWARDS.

Do NOT run the system with R22 only. It will not circulate the lubricant properly and all bets are off as to what would happen with pressures.

The idea of the R22 and nitrogen is totally for static testing with an electronic leak detector. By making the pressure with nitrogen, you have high pressure throughout the entire system to force the R22 through even a small leak to be detected.

I have my doubts as to how effective straight R22 would be due to lack of pressure.

Good luck,