Poly will yellow over time, its a fact. It will yellow very quickly if its exposed to sunlight. 2 part automotive clear is the only "right" way to do it. Anything less is a shortcut.
If you do not have the place or equipment to do this type of work you would be well ahead of the game to pay someone else to do it. I have a black knight that someone cleared that looks like crap. If I had to guess they used a foam roller because you can see those tiny air bubbles that you tend to get from them rollers. Its a real shame because it was otherwise a decent playfield. Now its going to take hours of time to strip the old finish off and prep it correctly for some real clear.
There are different ways to do many things. In some cases one may be just as good as the other. When it comes to clear coating a playfield there just is no substitute for a 2 part auto clear. Sure you can get decent results with poly but why take the risk of it turning yellow and the fact that it will not hold up to wear as well. The amount of work is the same for both products. If I take all the time to prep the playfield it only makes sense to use what will give the best results and protection.

I dont know of any epoxy or polyester resins that are truly UV resistant. You would typically use a gel coat as a top coat to protect from UV. I wouldn't recommend epoxy as a clear. Its tough enough but like poly it will yellow.
Btw, west system epoxy is the shiznit.