You're correct, it's bumblefoot and it needs to be removed.
Soak her foot in warm epsom water for about 20 minutes. Then snugly wrap her in a towel with her foot exposed. You can tie her legs together with a shoestring or a piece of clothesline to keep her from kicking.
Use a razor blade or scalpel to slice through the scab to open it up. There may or may not be blood, have paper towels standing by. Then start squeezing at the base of the bumble forcing the infection up and out of the opening. Dont be afraid to squeeze it out as hard as you can because you want all the infection removed. It will be solid and kind of cheesy looking. Keep squeezing hard until it's all removed. Soak up blood with the paper towels as needed and then squeeze some more.
Once the infection has been completely removed, apply betadine to the hole and pack the hole with neosporin. Apply neosporin to a gauze and place it over the hole. Then use duct tape to wrap the foot, cut it in strips to fit between the toes and legs as needed, not tight though, and not loose.
Unlike vet wrap, duct tape will protect the footpad from moisture and wetness.
Then release Magpie to go about her business. She will try to remove the duct tape to no avail.
Wait 3 days, then remove the duct tape. Use scissors to slowly and gently cut the tape vertical up or down on the leg first, avoiding the spur and toes. Then simply peel the tape off the leg and foot. Use the scissors as necessary.
Inspect the surgery site. There shouldnt be any redness nor swelling and should be almost healed.
Then put neosporin over the hole, same for the gauze and use duct tape again to wrap it up as before. Then wait 4 days, remove everything and it should be completely healed.
Magpie will be good to go.
If there is redness/swelling, it means you didnt get all the infection out of the footpad and you'll have to start all over with soaking the foot etc...
It's best to get it the first time.
I've done quite a few bumblefoot surgeries and only had to repeat the surgery once.