I think my gearbox cooler is leaking - having washed down the radiator and two coolers, I seem to have a bit of oil reappearing on the bottom of the two coolers. Need to sort this before putting the condenser back on and filling the A/C up.

I've got spares on my breaker, but I've read that these tend to not come off too easily or at all without breaking the ends off. Any tips on how I might remove the pipes without damage? Plusgas? Heat? Prayer?

See if you can wrangle the cooler and pipes complete off the breaker. It's the ends where they attach to the cooler that seize up. If you can get 'em off without having to separate, then that'll prove you can get them back on :-) Failing that, heat might do it, but it's a lottery with softish parts...

You know when you have put too much force on it when you break it.
Heating the nut will partially work because the aluminium thread inside will expand more so what you can do is loosen the pipes on the other end, hold them up and fill the cooler with water and then heat the nut with a torch so the aluminium inside stays relatively cool.
Just a try.

I once tried to cut through the nut to get a pipe off my diesel gearbox cooler. The idea was to cut a slot along the length of the nut then open it up with a screwdriver. It took ages and didn't work. Maybe I am too clumsy. In the end I bought a new cooler and new pipes.

To prevent the new one seizing up again, I used lots of copaslip on the threads and wrapped denso tape round the joints. Because it is two dissimilar metals, aluminium and steel, it results in electrolytic corrosion in the presence of water. Keep the water out and it shouldn't corrode is the theory.

Hmmmmm

I'd probably be able to get the whole thing off with pipes on the breaker, but doing the same on the other car without removing a load of stuff might be a bigger challenge.

I'll have a go at getting it off, might get lucky.

If you can get your good cooler and pipes off intact, pop down to your local Pirtek with it. They'll put you in some dismantleable joints in a convenient place in the pipework. Might even do it for a tea fund contribution if you catch them at a slack time. They did this for me when I went in to get some HP PAS flexi hoses grafted onto my old pipes to save a few pennies!

Hmmm! We do have a Pirtek in Poole. I'll have to investigate.