You are right, if the sender does an RBF on the transaction then it means the first transaction is invalid because nodes will have replaced the transaction with the new one and the receiver can’t do a CPFP on that particular transaction again. As for the balance in don’t understand what you meant but it wasn’t even added at first instance.
It wasn't added as a confirmed balance. If you use Electrum, as soon as the new transaction shows in your history, the amount also gets added to your balance but it remains unconfirmed. I don't have any unconfirmed transactions at this moment, so, I am unable to show you any screenshots at this moment. But, it adds the amount to your balance even with zero confirmations.
Okay, I did a Google search and found a screenshot that could give you some idea.
In this screenshot, the guy received 0.12807mBTC and it's still unconfirmed. But it is showing in the balance that he can do CPFP if he wants.