It’s possible to skip the mempool, by including it directly in a block, giving no time to a QC attacker. Not worth it right now, but if QC was known to exist it could be done.

Is there a way to guarantee that this block doesn't become orphaned? A transaction isn't truly committed to the blockchain until it has several confirmations.

There’s no way to guarantee absolutely, but you could wait to announce until you got lucky with a couple consecutive blocks if you had a lot of hashpower (pool operators).

It would be an expensive endeavor since it involves throwing away solved blocks, but could of course package many QC safe(r) transactions so probably a profitable service in a QC compromised era.

Do you mean by mining the block yourself? Or a really high transaction fee?

Well, not necessarily yourself, but through an entity that would not gossip the pending transaction. If it paid well any pool or mining farm could include it. A high standard transaction fee would not help.

Yup that is correct but that window is reduced to a minimum. Your adversary will need to crack your PK in 10 mins and submit a replacement transaction which may or may not be accepted.