I think its a bit naive to think that ATACMS will be the war ending capability. It certainly will increase options, reach and capability. I think it would have had a bigger effect earlier in the conflict. Still I'm sure it will be a very welcome addition to the arsenal if they get it, though I do feel even if they were to receive ATACMS, they wont receive them in sufficient quantities to have hugely significant effect. If anything it will be a huge hinderance to Russia as they have to pull vital equipment and stores further back yet from the front.
Theoretically yes, however it would have to be static. Would be highly risky and a huge waste if it misses though