I wouldn’t beat yourself up over it, really. Surgery is done in the most pathogen-free environment as is achievable, and still people get infections following procedures. So long as the needle was sterile, you gave it the best chance. Topical antiseptic would hopefully prevent infection from the “skin” from being introduced, but would do nothing if you happened to knick something internally.
I find this thread very interesting and I commend you for doing what you’ve done. I really hope it works because I agree, the fish would have likely died without some type of intervention. I’m anxious to see how this works out but no matter what I think you did the right thing.