I'd go directional with the M. I think if you use an omni, you are stuck with a mic angle of 180* but can change the pattern from omni > subcard > card depending on your mix.
Correct, thats one of the advantages of the subcards, is that you get close to the omni pattern, but you can still get some directional tweaking out of it in post.
I'd run mk21/mk4 with the mk8 and run the 41s as backup
That would be my suggestion as well.
If you want to bank on your M/S setup being correct, depending on the mount used, you could stack a 41 on the bottom, the 8 above that, the 21 above that, and the omni above that. The caps are small enough that if you don't use a windscreen, I don't think you would get much of an issue with the spacing of the omni to the 8. At Hampton we stacked LDs one night in a similar fashion to this and it didn't introduce major phase issues.
edit: if I was going to do this, I'd use a kwon/louie-style MS bar for it and not a delivered setup (I know on a neumann MS mount, there isn't a way to stack caps). Probably take some tweaking anyway, but it's an interesting idea to play with (IMHO).