Hi Adam,
Have you thought about mechanical issues? I would have thought the only thing the aircon would do is put a load on the engine. It sounds like you don't have a smoke machine so might be worth trying to borrow one (one of my best investments!) just to double check for leaks. Have you done a vac test with the engine running and/or comp test? Is the STFT just a default value? I can't see that being correct if the engine is hunting but someone may correct me if I'm wrong. LTFT at 8.6% does seem high but probably not enough to set a DTC and possibly settles when under load? Have you got any way of monitoring injector current and/or pulse width?
Sometimes we can't just give customers that immediate answer and if they are reasonable they will understand. I always tell them that work in the diary gets priority.
Hopefully that's given you some ideas on the next step.
Bryan.