Oh I feel your pain. The sub Q thing is scary, had to do it for a cat of mine that had cancer but up by the shoulders where a momma cat would grab them is the best place (as I'm sure your vet showed you). Just grasp the skin as if you were picking him by the scruff and slip the needle in and massage a bit. I always gave mine a bit of Haagen Daz vanilla ice cream afterwards (don't know if that's allowed for yours though) just for some positive reinforcement.

You love them so much, you try to do your best for them and vacations are over rated; that's what I tell myself anyway when we have an animal crisis. lol

Hugs and good luck.