Having started this thread I thought I should add a late update. I gave up on Sunday having hit the same block after restarting 3 times but unable to see my error.
Then I remembered in the early hours of Tuesday night that when I identified our special numbers a couple had more than one possible pair of squares as factors and I was using the wrong pair for 65.
Started again yesterday and slogged through the maths. The end game was very neat and overall it was a rewarding if grueling puzzle.
I have huge respect for Elap. Constructing one 5x5 grid with a dozen 5 letter words is remarkable. Constructing a second with the same letters is astonishing.
I can't imagine how to even start on constructing the mathematical element.