Your Sienna will not get you to the Frenchman TH, but close, within a mile. The Colo Trail crosses the FM trail ~1mi up-creek from the TH. So fundamentally your loop is sound.
If you have twenty-something legs, no reason you can't do both Harvard and Columbia from upper FM on day 2, they're both right there. Even do one on the day one evening if the thunder gods are smiling, as we speak Thor is very angry. Then the south PC trail to PC and upper Missouri Basin. Then loop back to your vehicle via the PC trail to the CT. Not a clue how well any of that is signed, I've become a big fan of GPS Topos
and use them exclusively. All the trails we've discussed are on those GPS maps in bold red dashed lines.
When you're on top of Harvard you'll be able to see the whole trip, plan to get there early so you can spend an hour to scout everything.