It sounds like it’s quite a difficult repair, especially under your circumstances, so you could try disconnecting the supply of vacuum from the engine and see if that works. Apparently the default setting for the recirculation flap is open, allowing fresh air in, so it’s fail safe and this is why I disconnected mine a few weeks ago, I’ve had no problems with condensation etc since then. When I disconnected the vacuum supply I cut the grey pipe in the engine bay right near the battery and pushed the ends of the grey pipe into small pieces of rubber hose which were blocked at their other end so the engine wasn’t pulling in excessive air through it, I thought this would be easier than trying to clamp the grey pipe as it’s made of quite a hard material and it would be difficult to seal properly this way. Good luck with yours.