I have done partial clear coating with varethane. Kept the games for up to 5 years and they stood up well.
I have used 2k auto clear but only on full playfields.
The varethane is not as clear as the 2pac. But is quick, easy and basically odor free. If you sell the game and the new owner freaks out about the varethane, he can remove it using xyelene without ruining the playfield.
Others mileage may vary

Yes varethane poly. Water base satin or semi gloss.
I did an entire Trident playfield in varethane. Had that game for 10 years and likely played it in excess of 3000 games. It looked fabulous when I sold it. It had some dimpling but no more than my volcano that was cleared with 2pac.
For small repairs I use the brush on. For larger repairs I use the spray.
I have also used in my airbrush to seal touch ups on cabinets.

Exactly.
Not every small touch up or every machine requires a full blown 2pac clear coat job.