OK, I will take it to a shop here and see if they think the threads can be salvaged at all.

In order to prevent this on future bikes, what should I do? Was it caused by water getting into the shell, corrosion starting, and then galling (basically the two metals fusing) occurred?

If you install a bottom bracket correctly, and then never take it out or re-grease it or anything for 10 years, would you expect this to happen or would it be fine?