Can I presume that any after market sights for MP 9mm will be ok with this 380?

I have no direct knowledge on this matter, but I sure would not make that assumption. The Shield rear sight is different from the FS and C. It’s narrower by maybe 1/16” or so. The little 380 is smaller yet, and I suspect the dimensions of the sights are different.

Do regular M&Ps and Shields have the front sight held on by a weird C clip arrangement like the .380 EZ? I would be surprised if they do. I have never seen that on a handgun before.

No shields have a regular dovetail.
Not picking up the EZ for a week or so.

As now there isn't any aftermarket sights for the EZ. I have been looking almost every day. I would imagine that there soon will be some on the market before to long.