Rusted nut on shock stud

[amputee]

In one line: What the heck is the easiest way to get the dang rusted-on nut off the top of the old rear shocks in order to get off the mounting bracket to put on the new shocks?

Background: Over 53K and my 2007 Aveo5 SV hatchback was at a disreputable chain for a brake job due to noise (they replaced the pads AND rotors, due to the rotors "rusting through"). They wanted to replace the rear shocks as said they were "leaking like a sieve." I declined, the bounce test seemed fine but the shocks did not look great and had a lot of mileage. Watched some videos and read up here about it, looked easy, so I ordered a couple of Monroe #5794 SensaTrac shocks from Rock Auto as my usual parts place didn't have Aveo shocks.

Put the back left on a jackstand, took the tire off and put the jack under the spring, with some difficulty and an extender I loosened the two 14mm bolts on the mounting bracket in the wheel well, but the 14mm bolt at the bottom would not budge, even though I had thoroughly oiled it days earlier. Went and bought a longer/better ratchet ad got all three bolts out and pulled out the shock. It did seem bad, not pushing back out when I pushed it in.

The mounting bracket is held onto the shock stud by another 14mm nut. This is not accessible without taking it off so I had no way to oil it in advance, but I added some immediately. Turning it with a socket or wrench just turned the whole shock body. Tips online included holding the flat end or the bar with a vise-grip. This was a joke: that rusted nut is on tight, and even putting it in a big clamp with tremendous force, using the wrench on the nut would just rotate the shock.

I finally hacked off the nut with a hacksaw, which left me in worse shape as the top mount was still stuck in place and there was no longer anything to turn. I ended up taking it to a shop and they torched the end off, so I could get the mounting bracket off and put the new shock in, but unfortunately I didn't have an extra jack or time to get the other shock off, so by the time I got the other shock out the shop was closed, I have to go to work in the morning, and of course the nut on the shock is just as immovable, if anything worse. I sprayed some liquid wrench crap on the nut before I put that shock back on hoping maybe it will loosen up in time, but I am not hopeful.

Seriously is there anything other than a torch that will take this nut off? There has to be a better way. Swapping out shocks looked like such an easy task but I spent five hours on it with only one new shock on to show for it and feel pretty broken up.

P.S. This is stupid but there was a wire looped on either end of the shock holding it together in the box, I took it off as it seemed to be obviously for packaging, but I haven't seen this mentioned anywhere and now the left side is noticeably higher than than the right, as mounted it is quite extended to the point where there is a small gap between the top and bottom half, I hope that is OK. I'm pretty sure I was supposed to remove that but I hope that the Monroe 5794 is the correct part for this car, it was listed as compatible...

Also how bad is driving with one new shock and one old/bad in back? It can't be good and I'll replace the other on ASAP but I hope it's not a deathtrap on the highway in the meantime.