Glad you were able to step back and not get hurt. Glad to hear a classic survived the fall and will be camping again.
Couple things I noted as I was reading through the post. I too have manual jacks which I use a drill with. I never go 2" per rotation, but rely on the drill to tell me when the jack is taking to much road. Then I will go to the next one and work around always making sure the front is higher than the rear. I will usually only work the two front and one rear jack but catch up the other rear every 3rd rotation or so. But I don't really think your jacking method was the root cause, just what brought the other issue to fail.
When you installed the front jacks, did you put a re-enforcement plate on the inside or just rely on the wood? The front jacks sees about 2/3 of the camper weight and that isn't shared equally when you are raising the camper. Just a few screws into wood generally isn't enough strength to hold the leverage and weight when applied lifting. This is why there is always a metal re-enforcement plate on the inside of the walls of the camper which the front jack screws penetrate so the wood is squeezed between the jack and plate.
We had a '69 Franklin with the 3 jack setup and I can still remember how nervous it made me loading and unloading. Never felt as stable as my Lance does even when only 3 jacks are on the ground. Think it had to do with the single center jack being at COG carrying 1/2 the weight of the camper.
Good luck with the repairs and enjoy some camping when you get it done.