1. Can a transaction with nLockTime be replaced with a new one that has a different nLockTime value? What happens if it gets replaced?
This is one of what timelock can be used for, inheritance purpose. Maybe you set it to be confirmed after one year, but you think of extended the time because you are closer to the time already, you can extend it and use RBF to double spend the transaction and you can make the new transaction a timelock transaction. But do not forget that you will always replace it a little higher fee each time.
2. If a transaction with nLockTime is stuck in the mempool because of low fees, can I use RBF to replace it with a new transaction that has a higher fee?
Yes. In case of inheritance, the person that wants to inherit it can use CPFP and use high fee to get faster confirmation.
There are pros and cons while using this for inheritance, so know the coins as well if you want to use it for inheritance.
Why not ask this on the general board where you will get more better replies?