Shower valve questions

Full disclosure, i have very little plumbing experience. So I may not know the correct terms for everything.

I added new shower handle trim to our shower because the old one was old and tearing apart. Anyway, the flange won't go all the way to the wall (see pic below). I'm worried about water getting behind it and causing damage. Is there something I can put behind it?

Also, I'd like to replace the part the trim kit goes over (I'm assuming it's called the valve) because it's old and rusted. Would I just use some vice grip pliers to do so? Just want to make sure before I break something and my wife makes me sleep on the couch for a month.

Here's the pic:

to seal that escutcheon to the wall, silicone caulk

the other item, confused as to what you are talking about

Fixing something that ain't broken is just asking to have a can of worms opened up, but that's only my opinion . Either way if you insist I'll help you as best as I could , take off handles , covers and unscrew the nut on the toes of the stem. Make sure you're water is turned off before attempting , then yes take some vise grips and pull like there's no tommorow.

Hold on. Looks like Price Pfister trim.
Is that trim the same as what you took off?

tack off one of the chrome escutcheons and handles and snap a photo.

Normally those handle will cover more of the escutcheon, So I think those escutcheons have bottomed out on the valve. did you change the stems or just the trim?