Find a steep hill in your area and at 3000rpm in 4th, nail the gas right down. If the clutch slips, there's your answer for question a)
My ED30 clutch slipped on the dyno at pretty much that same mileage - at exactly 300lbft - after a remap. So long as the roads were flat and I avoided working the engine too hard by pulls in 4th and above from low-ish road speeds, it was OK.....but you know how it is......you fancy a Saturday afternoon hoon and you get clutch slip....annoying!
Question b), you'll need to speak to APR to adjust the N75 boost request in that rpm region.
The Helix clutch is the best option. It's a bit heavier than a worn standard clutch, otherwise perfect road manners. No juddering or nasty chattering noises. AKS Tuning do them for a good price I believe. You will need a new DMF as well unfortunately.
TFSI tuning is expensive mate. If you already have that attitude towards it, without trying to sound discouraging or blunt, I would seriously consider selling up now before the costs escalate....and they will if you continue down the modification path
And 83K is just a baby. Mine's approaching 110K. 40K of it remapped. They can handle it in their stride if you keep things sensible
The clutch isn't slipping currently, right? So maybe tackle your other priorities first and come back to it later?