I highly doubt that your first attempts to level your trailer caused lasting problems. I try to level my trailer side to side by backing onto homemade wooden ramps I carry and then front to back using the tongue jack. That said, I've been in some screwy situations and have on numerous occasions used my scissor jacks to the point my wheels were turning free, and I don't think my Forest River trailer frame has ever been damaged.
As you noted, though it is possible to wrack the whole trailer, but don't think for a minute this hasn't happened crossing rail road tracks or pulling in and out of gas stations. My guess is your axle is bent and needs to be re-straightened. I had my axles realigned right on the trailer when I had the same kind of wear after the front axle lost it camber.
What scares me is the front of your trailer sounds like it may have dry rot or structural failure in the front if it's moving independent of the frame. I would be highly suspicious of the the prior owners motive for trading it off and the dealer for selling it if they knew it had problems. I do hope you didn't buy some one else's problems, but hopefully you can locate an honest service person to help you figure out what's going on. I would not take the blame because of your previous leveling experience. All the best, Hans
