If machine guns don't fatigue on OP fire, including the supporting M-gs, can supporting MGs later then shoot "actively" at the same squad if it moves again? CAn an MG first support and then later be the active MG on the same target in a new hex?

No, p.32, "If a unit is capable of firing multiple Op Fire attacks.....it may only make one attack against a single enemy unit during that unit's activation...or participate in combined Op Fire....."

one shot pr. enemy unit only.