To me, it looks like the coelomic cavity protruding. I would take great care cleaning that area. The vet may be able to stitch the opening, but the maggots need to be gone.I think? Light shines through. Because we weren't here, I don't know if that was there before the flies and is unrelated, or if its resulting from the flies.
If you have wound spray, then I would spray that area, if not, then warm your triple antibiotic ointment between your fingers and apply a thin layer on that protrusion.
The other areas, you can apply a thicker layer of ointment.
I would not use pine tar for this application/treatment of wounds. Too sticky, the skin is already fragile and the application of tar is likely to damage it.
id probably try Stockholm tar, we use it for fly strik on horses but its also good for wounds . I just dont know what the welt/boil is but it may help with that as well?