Brian,
A dumbbell tub wrench should help in removing the lower drain fitting
I'm not much on repairing rusting tubs. I would much rather replace them. If I try to "repair" a rusted tub, they might expect it to hold water. Do you know how many pounds of water there is in a full tub? I don't.
You can try Silicone, and letting it cure 24 hours.
Terry