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

If it is frozen solid at 20F, its water, not coolant. Someone added an incorrect mixture. If it only hit 20F for one night, its not likely a deep freeze that would crack your engine block, but that is a risk you are running until you get the fluid drained and replaced. Your radiator or expansion tank may have cracked. Only time will tell

Its not that challenging to DIY. Tools are fairly cheap or could be rented. I’d recommend new rotors too

Yes, the damage is more than its worth if you have to pay for professional labor. I’ve repaired a few myself and thus did not pay for labor. Parts aren’t very expensive for E46’s anymore. If this was a convertible and manual, I’d offer you cash now as I am on the hunt for one.

Your best bet is to part out what you can and scrap the rest. You may get a 1-3k that way depending on how good of condition the parts are in

It may be repairable, but you will need the tools to do so. Did airbags deploy? You may be able to part it out. If you have the sport seats in goof condition those may be desireable. You can sell off other parts before finally scrapping the car.

Alternatively, try to sell it whole. If it were a manual and a convertible, I’d be interested. I’ve fixed accidents like this before

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