At this moment, I will see 1 confirmation of this transaction in my wallet, right?
It depends which chain you will see. If both, it depends on your wallet, but I suppose it is better to show less confirmations than more if there are possible chain reorganizations.
This means, my transaction will go back to the mempool, right?
Yes, in the same way as all other transactions from that block that were missing in the winning block, except coinbase of course.
So in my wallet, I will see this transaction as „unconfirmed“, I guess.
Yes.
Does that mean the settlement transaction will be undone and the coins re-appear in my lightning channel?
Yes, Lightning Network transactions behave exactly the same as any other on-chain transactions.
Can I always do a RBF (replace by fee) in this case?
It depends if that transaction was marked as RBF before. If so, then you can safely include more inputs and do RBF transaction by keeping previous inputs and outputs intact and adding new inputs and outputs to increase transaction fee. If no node remember previous transaction, the situation is the same as it would never be broadcasted, but it is difficult to be sure that every single node discarded it, so to be safe you should assume that some nodes still have it. You can also rebroadcast exactly the same transaction, especially if mempool is less congested than before.
