Your list of often-smoked tobaccos isn't a hit-parade of biters. I rule out tobacco brand/blends as a reason for tongue bite, especially with PS LBF.
Now, two hours from two coins and a swampy heel + wet dottle... this sounds like a lot of over-suckage against moist tobacco and/or a restricted bowl or partially plugged stem. It's hard for me to get 30-minutes+ of smoking through a clear pipe with one LBF coin. LBF, by the way, is something I always buy bulk, it is never too wet (at my local B&M anyhow) and nearly always smokes to the bottom of any pipe. I like to tear those coins apart, sort of the centers and bury them mid-bowl for a sort of cavendish parfait thing, by the way.
So, to brass tacks now...
Do you swab out a stem/heel with a fluffy pipecleaner mid-smoke (or thumb-over the chamber and flick the stem) if ever you hear a gurgle?
Do you fold/stuff coins or rub them out?