That doesn't sound fun
Hope it all goes well!
As for combining the MC/Open, there a lot of reasonable seeming ways to do it. Assume 9 MC teams, 5 or 6 Open teams.
1) Seed the top 2 (if 5), 1 (if 6) 'Open' teams through to the Open 1/4 finals. Everyone else plays a prelim round of 6/7 matches, Open vs MC where possible.
Rnd 1 winners into the Open, losers into the MC.
2) Seed the top 4 open qualifiers. Then have two simultaneous qualifying rounds:
a) 'Open' - those top 4 teams vs 4 other teams. Highest qualifiers? Those who want to?
The winners in these matches through to Open semi, losers a place in the MC quarter finals.
b) Everyone else in the MC plays qualifiers to drop down as required to get balancing numbers.
(2) is an effort to build in a real carrot, while also letting the marginal MC/Open overlap teams have a 'free' shot at a surprise win. Like Somerset vs Yorkshire in 2012
This does requite one more round I suppose, which might be an issue. Might be helped if the semi/final's got rolled into a weekend? I think there's definitely a potential case for considering that anyway. People are very used to weekend chess nowadays, and it'd make the travel to chess ratio in the knock out stages very much kinder! The 3rd/4th place play offs might be gently annoying.