Merrimack is very solid. The plus for Miami, and where they have the biggest advantage, is they are a veteran team that has been to two straight frozen fours. Merrimack hasn't been to the tournament in many years so it is a new experience for all of those players. the moment may be to big for them. Merrimack can score in bunches too, I saw them play on tv against UNH and they out played them, would be a great matchup.
As for INCH.com's rankings, they are the most realistic rankings out there. They take into account how you are currently playing along with your record and results. Miami is a better team than both Michigan and Notre Dame. Both may be ahead of them in the standings but I chalk that up to, and I would assume they do too, Miami hitting a slump in the middle of the year. But outside of them being ECAC homers they do the best job ranking teams appropriately.
Not sure how the ND/OSU series is a catch 22, its pretty straight forward. GO ND! Osu can very easily drop out as a TUC with a split, but Miami is far better off having OSU out as a TUC than in. If Miami sweeps lake, it doesn't matter what ND does because Miami will be fine. Having OSU in as a TUC hurts Miami against ND and Mich because of their records against them. You never want a team who has beaten you twice to hang around as a TUC. Ideally you want NMU to stay in, OSU to drop out, and Miami sweep. If that all happens and half the other teams I listed below win Miami makes some big leaps in the PWR.