I fly PA44's regularly.
From memory, at 88kts the dead engine windmills at about 1400 rpm, give or take a bit. It makes a small difference between mixture ICO and engine idling, but not much.
Have done several VMC demos in the last few days - and I think the dead engine was windmilling at a little over 1000. But that was idling, not mixture ICO.
O8