with the AAC, MWC, etc.

I don't think they (the ACC/MWC) care (legally) what the B12 does or doesn't want to do, or what the B12 wants to say to them. Contractually, those teams that might leave all still will be required to provide notice and buyout. The timing and fees can be negotiated away by the teams and remaining conferences in exchange for other concessions, and to the extent B12 helps with the fees in that buyout, I suppose there is something there.

But, I doubt that the B12 says its own teams can't play MWC teams or something if the MWC teams hold firm on contractual terms. I don't think the B12 (or any P5 conference) has weight/leverage, etc. to interfere successfully with contracts between the MWC/ACC and their respective members. In fact, there are pretty strong legal reasons why they might have to just sit it out and wait for the schools to resolve these issues with the conferences individually.