Normally when node peers communicate with each other, they will agree on blockchain fees periodically. What can happen is that if the last time the blockchain fees where agreed is when the blockchain fees were high. Then let's assume (because it was a force close anyway!) that one of the parties became unresponsive and then the force close will happen with the agreed high blockchain fee.

As per the "RBF", this is tricky because replace-by-fee creates a new transaction id and this new transaction fee may be unknown to the lightning node(s) in question.

LND handles bump fee with "child pays for parent" so that the original funding transaction goes through no matter what. If you were to bump a funding transaction with RBF then your channel will not get opened at least by your counterparty because they are looking for the original funding transaction to go through. For close it's surely the same, now for Force Close, I agree, it could just RBF but that's all really above my pay grade here :-)