Stressless,
I don't think this has anything to do with NP pond. I would urge you to seek to properly line at least the known problem areas in the way esshup suggested. I might even give esshup a call to see if he might be willing to oversee it. If your resolution is in your ponds and on your property then you aren't depending on anything and anyone else. It's not likely anything you are able to convince your neighbor to do will help your situation. I would focus solely on how to make a tub of my own ponds.
IMHO, something changed subsurface and this why so many spatially separated ponds are affected. I don't think the neighbors dam break had anything to do with your pond's failure ... even though they may have co-occurred. Probably ... everything is connected to some other event. My sense is that your pond spilled down to the leak and from there it went DEEP. You pond is a tub up to that point. In order to be sure that the pond holds water ... you must at the very least patch the spill point such that it is no likely to fail soon. I don't think this will require a cement plug but then maybe I am wrong about that.
