A single UTXO double spend on two chains is possible.
Is this the case?
Correct.
What is the difference in the two transactions?
I linked it with my post.
The fees? (after an initial claim of an RBF transaction being bumped up, it appears the was not the case)
One of the transaction actually do have a higher fee and that makes me think that the transaction that was on the losing block was actually the RBF'ed transaction, since the lower fee transaction actually signals optin RBF.
The recipient address? Well, this would technically be a double-spend. But is this what happened?
The transaction had an additional output that was spent to a P2SH address and a few different UTXOs being spent as well.
I guess this was intentional.
No. Both pools mined the transactions within seconds apart, and it's likely that SlushPool didn't manage to replace the existing transaction with the RBF transaction that spends a higher fee. Unless the attacker owns either of the pool, it's highly unlikely that it was planned at all.