You are NOT stupid! Stop blaming yourself! Things happen, to the best of them! We have all been in some similiar situation, I am sure!
I wonder what type of injection they gave him? I am sure it will work!
As for keeping them overnight...I don't know...I strongly stand firm that they come home! Even when Mojo had those problems that I PM'ed you about, he was on IV for 7 days and I got to bring him home every night...he would go in every morning, stay there all day on IV and come home every night. They kept the IV thing in his little leg all wrapped up.
Course, my vet knows how crazy I am...and we have a very good relationship. Every night, I would keep a list of every time he got up, if anything "came" out and what it looked like, if he drank or ate and how much, etc. Every day, I had a list for them!
I just think they do better at home, where they know they are safe and comfy..not in a little cage in a strange place, with strange smells and loud dogs and no one there! Poor things!