I've done a couple of these, I had good luck with patience and a good penetrating oil (P Blaster). Basic thing to remember is you're trying to unstick two things at once. 1) the flare nut threads to the raditor fitting and 2) the line to the flare nut. The other thing to remember is that the car works now, and rushing the job may make it not work ;-)

Spray both ends of the nut liberally over several days. Heat occasionally with heat gun (or even a hair dryer).

Put a wrench on the large fitting into the radiator and a good fitting wrench on the flare nut. The line is pretty thin and could kink or flatten, so I don't try to hold the line with anything stronger than a hand.

If you can break the nut free, work it back and forth 1/16th of a turn at a time. That plus more heat and PBlaster will almost always work the line loose from the nut.

I wouldn't cut the line. Shouldn't cause any harm, but sooner or later you'll need to replace the radiator and you'll have to get this fitting off anyway.

Re: the drain pan, I don't bother when flushing. The oil pump in the transmission is pretty good at getting everything out, and I've heard of folks stripping the oil drain plug.