As I wrote, some duck people have found more help on the duck forum than the emergency forum, but that is not always the case, and for this question, I think you get a more broad range of experience.
Please keep us posted! I take mental notes in case I ever need to do some of these procedures.
I have seen vets inject under the wing, but I am not an expert. Let's see if someone can check in soon.
See if this helps
https://www.backyardchickens.com/t/352701/how-do-i-administer-a-penicillin-shot-to-a-chicken
I would get my boys back, and begin treating her for bumblefoot. The bath was an excellent idea. Get some triple antibiotic ointment, not the kind with pain killers. If you can afford a vet you may want some help, but many people successfully treat bumblefoot at home.
I avoid cutting feet...