Thank you for your reply.

I think that I got it now, just one small remark to my initial post: In my post I wrote that the transaction will go back to the mempool after the chain that included my TX "lost".

I think this is not 100% correct, because for the "winning chain" it never left the mempool.

So in worst case for me it will look like it was send back to the mempool and in worst case "1 confirmation" will change to "unconfirmed" but in reality nothing changed, it was just not included into the next block.

I think I got it now