How can I tell if a faucet valve seat is removable or not?
You have to look for notches in the seat wall to see if it is removable:



The seat in the picture has 4 notches and would be removed/installed with a common tapered square seat wrench.  Some newer seats will have a hexagonal hole in the middle and can be removed with an Allen wrench.

Look closer at your seats for even the hint of the square notches.  They can corrode over and be less visible, but from what I see in your pics, those are not removable.

For non-removable seats (and even removable ones), there is a "reseating" tool for faucets that is basically a round grinding disk on a T handle that can be used to grind down the seat a bit giving you a new smooth surface.  It is possible to grind too much, so you'd do a little at a time.