Reserve is to make sure the opener has enough balance on their side to allow for the commitment fee to grow. You have to account for the tx size of all outputs which can constantly change, anchor + balances + inflight htlcs, x fee rate.

FCs happen when one party deems their balance as not safe. Usually this means a htlc has not been settled and expired. This could allow anyone with the preimage to steal it. However, each lightning implementation has criterias for going on chain. The massive fee change could be seen as an attack because if the commitment fee is too low, the mempool could be full where the commitment tx fee would be below the min relay fee rate. This could prevent the FC making it to the miners so neither side would be able to CPFP.