I definately wax none-CCed playfield regularily (based on games played). It makes a huge difference in how the game plays and looks.

For CCed playfields, I'll wax them once in a long while and then use Novus 1 for regular cleaning. Waxing a CCed playfield doesn't seem to do much for it. I doubt much wax actually remains on it after polishing to be honest.

I do also wax metal guide rails and such if they contact the ball. Shines them right up.

I use carnuba wax.

I'd highly recommend waxing the entire playfield before you reassemble. Much easier, nothing in your way, and no need to tape the posts, etc. (well, at least for the first time you wax). And wax the mylar along with the rest, fills in scratches etc. and certainly does no harm.

Not entirely sure, I'm still using the tub of Johnston Paste Wax I bought 15 years ago.

It won't hurt it but some people have reported moisture accumulating under their "floating" PF protectors after application. It has been theorized that moisture in the wax could evaporate and condense under the PF protector. Other theories are that in these cases the clearcoat may not have been fully set causing the problem. To be honest, I'm not sure if there's any merit to these claims, but would probably not put wax down immediately prior to adding a PF protector to avoid any possible issues. I might wax the PF and let it sit without the protector for a week or two though before adding the protector.

If I am adding mylar, I would always wax first as it makes it much easier to get the mylar off later without taking paint, etc. with it.