She's right mate, no dramas. The pax are none the wiser and they'll only remember you for your silky landings anyway
I forgot to agree with you on the fuel imbalance problem too - ta. All fair and vaild reasons to have the CTR's off when departing with 10.5t or less of go-juice onboard.
I've only flown a sim with AUX's and luckily it's not too hard to manage. The autopilot manages nicely to trim in the event of a "oh I forgot to turn the AUX's on" and although we didn't burn much more than half the CTR tank you could see the trim was a couple of extra units nose down. The nightmare is for the ground crews I gather - refuelling with AUX's is a right prick of a procedure
From memory I think the CTR tank has to be full before you can pump into the AUX, but if you do something crook with the de-fuelling/crossfeed (?) valves (and have fuel in the wings) you can put the aircraft in T/O attitude at the gate
Dunno if the instructor was kidding or for real, but it did keep the session interesting.