Wife's G4 just had the battery light come on again after 2 years.

[Fummins]

Right back to square one. It might be the engine replacement did this AC in.

When car is in motion, park, or neutral I can hear it cycle on and the cabin is cooling off.

When idling in drive with my foot on the brake, it starts to blow much warmer air.

My mechanic has given up at this point.

What in the world could produce these conditions?

Improper refrigerant level would be my guess. I would give up with your guy and take up 7milesout on his offer.