I repair my own helmets as technically there's not a lot to them.
I can't comment on the value whether it decreases it but as you would know most helmet collectors are purists and don't like them to be cleaned.
Saying that i can't live with a post war damaged pot so i do what i can to conserve and repair.
The feet will need to be removed, I'd remove them slowly with a dremel and cutting discs as I have done similar before. By doing this you will also be preparing the surface for new welds.
Don't weld unless you have the experience as a novice will blow a hole