I had the same symptoms as you have,i got advice which led me to replacing the flywheel,like you...i found it hard to see how the engine mounting would cause the fault.I investigated misfires which led me to a dead end and then i replaced the dmf(reluctantly due to the cost),my engine mounting code then cleared and never returned,maybe your customer could let the problem progress until it gets to a point where the it might be more obvious.
I hope i am not leading you down the wrong path.
Cheers,
Dave.