Will a MOE pistol grip fit a 15-22A1 or is the MOE grip used on the 15-22 MOE different? TIA.

The lower receiver for M&P 15-22s uses milspec sizing for the pistol grip attachment, so pretty much any milspec pistol grip will fit. Some of them might not blend into the lower as well as others ( MIAD leaves a gap, from what I've read).
In short, yes, the MOE will fit fine.

There is the small issue of the rear takedown pin detent spring being held in by the grip. This is unique to the 15-22 vs. the standard M16/AR15 platform.

I didn't notice any difference between the MOE grip that came on my 15-22 and the one a friend bought for his, however.

What is the issue with the rear takedown pin detent? On regular AR's it's held in by the end plate but the 15-22 doesn't have an endplate. Does it get held in by the grip like the safety selector detent?

yeah there are 2 springs going to the grip one for safety and one for takedown pin.

the miad can be installed with no gap smooth like the original grip. the moe will have a gap at the back.