So I've IDed the Stem (Price Pfister), got replacement seats along with a seat gauge tool to verify with 100 percent accuracy that I've got the right replacement seats, got the straight steel seat removal tool with wrench (the straight tool has a more gradual taper than the L-style tool, so it should bite better), and will insert and tap slightly and then turn lefty loosey to remove (first applying gradual force to avoid stripping, and if it fights me too hard then turn very slightly to tighten to break it free, then loosen).

Now,

1) Before installing the new seat, should I apply a coating of pipe thread compound to the seat threads? Richard Trethaway from "This Old House" does this in a youtube video.

2) I've read in some sources that the seat is designed to not require any sealant on the threads, which to me begs the question - being that seat threads on valve body threads is metal on metal contact, doesn't metal to metal contact nearly always require a sealant of some kind? Or maybe the brass seat threads deform when threaded into the steel threaded valve body and it seals similar to a compression/ferrule connection?

3) Should I just hand tighten moderately, or with "extra ompff" as they say?

You can use a sealer, I never did. Tighten with extra "oomph" or the seat will unscrew when you turn the valve.