Ok got a few Questions on this
Pretty sure I am going with M3's and actually have to because the ONW I'd probably have to tie wrap up.
So, heard a few comments over the years you cannot lay a speaker on it's side up front if using it for a center.
for multiple reasons.
So, why not get a VP100 and mount four of them as ceiling heights, since that's the natural orientation ?
Would these not be better than M3's ?

Secondly, if not and one does mount M3's which way does the tweeter align, to the middle of the room or outsode walls ?
Do you mount then up and down or side ways ?

I'm upgrading to the M5HP and was curious if I am mounting the m3 whynot go with VP100s'

Even still if I keep the M22's I have now would getting four VP100's for ATMOS not be better ?

My brain hurts...lol

In ceilings M3s...be done with it. They sound fine and you can aim tweeters towards MLP. Can't install in ceilings?

I have found when installing my inceiling M3's recently it has a 2 gimble adjustable system. the tweeter itself can be adjusted and the circular bracket it is connected to also can be adjusted in the opposite direction give you many options. One note the outer ring if you angle it to its max you can't put the mesh screens back on without them being uneven.

Yeah, go in-ceiling if possible. I went on-wall, but I soundproofed my ceiling and didn't want to punch 4 holes in the ceiling (and I would have never found a good spot that didn't hit a ceiling joist or my hat channel).

Tweeter to the inside is the way to go in that case.

Can you put some other speaker up there? Sure. Heck, if space and money was no limit, then put the biggest speaker model up there and match all of the other speakers. Nobody is going to do that with a large tower style speaker. A speaker the size of a center channel just start to look "large" and "bulky" up on your ceiling.