Definitely one of the greats on permies, been following along for a number of years now, first pond definitely raving sucess!
As for second, I definitely think conical shape helped with quick seal, I also seem to remember you had a gallon per minute overflow or something going into their water trough, maybe constant water flow helped, removing larger stones as you mentioned also and I would think tighter fencing may help encourage more concentrated activity
I sometime get wood chips from tree companies that are mostly green leaves, maybe a load like that would help?
Feeding wet food scraps in the area?
Digging a small conical hole in the center (hand dug scale) to jumpstart things?