First, the MF only counts the person making the killing blow. Someone could be standing next to you with 0 MF or 10,000 MF and it wouldn't matter. Only the person making the killing blow. If the killing blow is from a merc then the merc's MF is added to the owner's MF. If a minion makes the killing blow, the owner's MF is used.
And for MF weapons on switch, I wouldn't worry about losing a couple of your minions. If you're about to kill the boss, you will probably be a couple of minions low anyway cause the boss will have killed a couple. If the boss hasn't killed any, then you don't have to worry about him overpowering your army. If you have a good MF on switch (hopefully a weapon and shield), switch close to the end and just respawn your lost minions after the boss is dead.
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Darzog