You don’t want to go under a car supported by a jack. That said, you should never need to go under a car to change a tire. I change tires using just a floor jack

Only time will tell but if your gram constantly went 7,000mi overdue, and ran it 3.5qt low on oil, its very possible that it consumes a lot of oil! I would certainly be driving around with a jug of oil and checking fluid level daily.

Once you know if the oil is junk, I’d start replacing transmission fluid, brake fluid, and coolant

There is no way to drain the fluid from the TC without removing it so a drain and fill leaves fuid in the TC. This is acceptable per the service manual. You could also disconnect the oil cooler line to get more out

Drain and fill is what is recommended. Flush often has detergents and those are bad