I'd buy a trailer rather than use a uhaul. It'll weigh less than 1,000 pounds easily. Sell it when you get to maryland and it'll probably cost less than renting. My friend in Carroll County used to use them all the time and buy and sell trailers all the time. Let me know what you get and how much, we may be interested in a rust free trailer.
As for the uhaul and flirting with weight limits:
1. Engine overheating is every Subaru's, 4 and 6 cylinder, first symptom. Usually when it sees steep grades, high heat, and interstate speeds. Seen it on many subaru's towing over the appalachians.
2. It's August - a terrible time for heat. Drive early and at night if she does it.
3. The CVT's are not forgiving. Change the fluid. It should be changed every 60k anyway.
4. Uhaul's aren't great for Subaru's. Uhaul trailers are very heavy, I guess they have to be for commerical/continual use and renter abuse. and I've had uhaul brakes dragging while driving. My guess it's either because they get beat to snot or the angle of them on a Subaru isn't typical and tends to load the system outside of expected norms.
5. Also keep in mine Uhauls widest uncovered trailer doesn't have a full open back tailgate area. So the 72" trailer can't fit something that needs the full 72" to load. Like a 72" lawn mower for instance won't fit through the gate even though it fits on the trailer.
6. Yes newer CVT's swap - I'm installing a 2017 into a 2013 now. Most are plug and play, some require just swapping some of the external hardware on top.
I wouldn't want any of my relatives to do it, but I might be convinced to change the trans fluid, monitor engine/trans fluid temperatures with OBDII device while driving, drive at night and take route 70 with tolls rather than 68 due to it's steeper grades.
...which may be why I'm installing a new CVT into my 2013 outback right now. LOL