I would prefer the way that Concacaf did the last WCQ round, where the top two seeded countries played a home-and-home on the final two matchdays. If the top two seeds do their job, and beat seed #3, then MD5 and MD6 become a direct knock-out of its own; it will be up to seed #3 to put the monkey wrench into it during the first four matchdays. That would set up a case where all 8 USA and MEX teams would play on MD5 and MD6. How to do that with only six timeslots could be an issue. Either you allow it, include Monday, or have three timeslots on Tue-Wed-Thu (6 pm, 8 pm, 10 pm).