Just a very quick update on Elvis and the Valium since it's been 3 hours since his dose.
It looks like no allergic reactions, and while the Valium takes the edge off the anxiety, it doesn't knock him out or sedate him too heavily. He has been interested in our lunch and on my doings in the kitchen, and while he still has some reservations about people (normal during/after) a panic attack, he's pretty well calmed down to normal. No drowsiness to speak of and his respiration looks to be pretty normal.
So unless something changes, it looks like he reacts well to the Valium, so it should go in my bag of tools for future problems.