Discounted Pink Fatty owner here.
V8’s on mine stay round as long as i i don’t go too low on pressure. Must try the pink floaters i have to make sure they’re round! Still running tubes but have swapped out the weighty O-O tubes for lighter Spesh 3″ tubes from Evans.
No problems whatsoever with carbon forks, have stayed tight riding on all sorts. You are correct, bung should be redundant once the stem is tightened, not sure why yours misbehaves.
My bottom bracket did come loose. There was(is) a problem with the installation as the length of the crank spindle is longer than the bottom bracket width. This allowed the plastic tophat to drift out of the BB on the drive side causing the spindle to be loose in the bearing. I solved this by making a 3-4mm spacer that slides in the spindle between the drive side spider and the bottom bracket tophat so its a snug fit with no gaps between the two crank arms. Incidentally i made the spacer from a piece of the carbon fork steerer i cut off and filed out a bit.
Despite the faff the fatty is my go-to bike and get ridden everywhere, a lot.