The insulation around the '21mm pipe' is standard. The pipe isn't 21mm ID it is a special design with one pipe running inside the outer pipe and 3 ribs supporting the inner pipe from the outer pipe, there's not much chance of cutting a rotten section out to weld a section in. If there's just been a small pinprick hole some people have bodged the outside of the pipe by using sealants, wrapping stuff around it soaked in epoxy and clamping in place with jubilee clips, etc. But usually if it's worn thin enough in certain places to have pin prick holes it'll be very thin on a longer length of the pipe than just where the pin prick holes are.
Usually if there's a leak it's under that insulation foam, you can slide the insulation up the pipe a bit to see if it's damp with AC pag oil. The undersealing on yours might make it more difficult to find a leak.
What seems to happen is the AC pipe is degraded by electrolytic effect - The AC pipe is aluminium, the vehicle body is steel, they are bolted together so are in good electrical contact with each other. Add an electrolyte such as water to the foam around the aluminium pipe and it effectively makes an electric cell with the aluminium pipe the anode and the vehicle body the cathode, the anode (AC pipe) is eroded whenever this electric cell is active which is whenever the foam is wet, especially if the foam contacts the hinged metal AC access door.
To minimise future corrosion you could do some of the following: Prevent electrolyte (water) coming into contact with the metal components, can't really prevent water getting on the pipe but you can paint the pipe if you roll back the foam insulation, with waterproof paint the paint might get wet but the pipe stay dry. If the foam is wet the point of most electrolysis effect will be where there's smallest gap between aluminium pipe and steel components, this is often where the pipe enters the flap because the flap presses against it at that point, I remove the flap and don't refit it on my own Elgrands to prevent this pinch point of most electrolysis effect. Electrically isolating the aluminium AC pipe from the steel body to break the electrical connection and prevent the cell / electrolysis effect isn't viable because the AC pipe is connected to the compressor which is bolted to the engine, plus the AC pipe is supported at various points by clips to the steel body.
I've found AC leaks before by pressurising the system with CO2 or even air from a compressor, then going around listening for hissing and pag oil leaks. Not really supposed to use stuff like air though because it contains water from atmospheric humidity and you don't want water in the AC system. But if you later get it properly regassed the vacuum test done before regassing should cause most water that gets in the system to boil off and be sucked out by the machine. If there's a leak though the fix won't involve cutting a section out and welding in a length of pipe in it's place.
My wife suffers from chronic pain too, it's debilitating - not nice to watch so I can't imagine what it's like to go through