I want to know how to play right:
If my Spacemarin in overwatch mode and he roll double dice - he get the jam. He can fix it as reaction on action of Genestealer only? Or how?
This mean genestealer can move for 1 square closer to Spacemarine without overwatch fire (because SM spent 1CP and GS action for fixing jam)?
Yes to all. Sometime the Genestealer reaches the Marine before he clears the Jam.
Also, Command Points can be used on ANY marines, not just the one that observed a Genestealer take an action. Perhaps the Marine killed the Genestealer when his gun Jammed. But, on the other side of the board, another GS moves, and a different Marine observes it take that action. A Command Point could then be used to clear the first Marine's Jam.