I haven't thought about it in a while, but at one point I was wondering if INP-26 should have been reversed:

Tag the delegator's npub as the sender, as if it was a normal note but add a separate field for the delegatee's npub. A naive signature check will fail (because the delegator wasn't the one who actually signed it) but a verification that's aware of the delegatee would sort out the confusion.

So then a delegated note would just require this one IF branch to be verified, and then indexing and relaying could remain identical to a normal note.