Honestly, the biggest danger from injection problems is cancer/tumours. If you're worried about your vet's casual attitude, I'd consider checking with another vet. It's not the vet's fault it happened (hopefully, usually it's just one of those things) but he may be worried that you'll blame him if it goes badly.
Steroids are usually used for scarring/inflammation, so there's a chance he'll give him a steroid shot. They're doing incredible things with "specific steroids" now, so the dangers are small that a problem will arise (I won't lie, anything can happen though).
I'd be wary of the pain med's, though. I'm against pain med's for animals unless it's an extreme situation.
It does sound like kitty is doing better otherwise, happily. The behavioral changes sound fairly typical and healthy. Let me know how it goes!
Thanks for your sensible comments. If anything, her appetite has increased since her initial injury... it's almost as if she's using food as a distraction from her pain. That's one reason I was worried, as a matter of course, that she wasn't also defecating regularly. (Before her foot accident she was more of a picky, slow eater, but her bowel habits were normal.)
It's a week since her foot's been healed, she's been off meds and had her enema. She also hasn't had to wear her head-cone/collar anymore this week (put on so she didn't pick at her foot and it also kept her from jumping around so much), so I was interested in seeing if these changes would improve things. Foot and appetite are great, but the twitching/bunching up/hiding/sore injection site remains. Spoke to vet about it today, and as I thought he would do, he touted the injection site problem as one he'd not encountered before and said my cat's probably just "sensitive." His only suggestion now is to have her take a painkiller for a week to see if it helps. I'll be going to vet's office soon to pick up meds. You mention steroids... do you think that is likely to be what he'll prescribe? Steroids scare me, are you familiar with any way they can be bad for the cat? What might the treatment be if the pain persists after a week of painkillers?
These med's will commonly cause digestive issues, and it can be a few weeks before they settle. You can try a bland diet, like chicken bits mixed with boiled white rice.
As to the injection site...unfortunately, sometimes injections can cause problems to crop up. Scarring, growths, etc... You can try warm compresses on the site if kitty will allow, but unfortunately if a problem is arising, you may just have to "wait and see" which problem it is before appropriate action can be taken (usually steroidal or surgical).