1) I had a friend who sent his M to the factory and they did it for him, it was many many years ago and they charged him like he was buying another M.
There might be a cheaper way to do it in US, Canada or Japan - they got awesome repair shops there.
2) I don't know, I shoot mostly 50mm with Barnaks
3) It depends on how much is gonna cost You a modded M3 finder and if You wanna go there.
4) yes, the life called large format, but some of us always come back to the good old Leica.