Thanks, Loge, for help a while back, which enabled me to complete the grid.
Now I'm pondering Muraria's tantalising entry 99 - what are the interlocking shapes, what are the extreme cells...!
Hi, it's the large (clipped) triangle that you're left with after you've carried out the first two erasures. The three blank cells are all part of the perimeter of said triangle. Read alternate letters of perimeter (24 in all) to get the names.
I got there. Alternative letters, all on perimeter of the main 'triangle' which isn't a triangle, yields two obscure and one well known men with a shared interest.
I'm struggling to even get started on the end game. What does "clipped" triangle mean? What are "3 like plane figures"? What are "64 east of....."? The only bit I think I do understand is the two little points reference. I could have that wrong, of course! Is Poat the "hare" man? Any help appreciated.