Since the original transaction and the malleated transaction have the same transaction id, the mempool will accept the first it receives, right? Will mempool compare the fee rate for transactions with the same id?
Ah, yes, that's right. Because they have the same txid, they would be treated as the same transaction even though the contents are different. So if a node received the malleated one first, it would ignore the correct one later, and vice versa.