That trick plus loads of grease to minimise scratches has always worked well for me. Currently have one of those 3ttt stems on the track bike, with the extra little grub screws that opens up the clamp a little. I'm surprised it's not a more common feature.
Did you try the "jam a coin under the screw, push against coin to open clamp" trick? Yes, I am late to that, sorry didn't think about it yesterday!
Was the only way I could get stuff through an old cinelli stem.
Sounds like you caught it all in time, oversize bar cracking and a stuck stem were on the cards. "thanks" indeed previous owner :)