Okay, it sounds like the consensus is to give her her shot at 6:30, but drop it back to 1u.
My vet told me not to give insulin if she hadn't been eating - but I don't know whether that meant AT ALL or right at the usual time.
Before today she's taken her meals very promptly.
Edit: she did eat a bit at 6:30! So I gave her 1u and she seems fine - washing herself (normal post-dinner Migoto activity).
I am going to ask my vet about trying a diet change plus 1u instead of no diet change plus 2u. Hope it works.
YOU PEOPLE ARE A MIRACLE.

Thank you from a nervous newbie.