impossible gameplay tactics on XL.

...how is the location of the Grail's burial site determined when the center is occupied? Does it form in a spiral? From left to right or vice versa? For example, sometimes I find it 4 steps to the left when the 3rd cell on the right is free...

I didn't quite understand the question. Is something or someone standing on the Grail, making it impossible to take? Ideally, it should be under a cross, which you need to stand on. If you want to find it in an invisible area, theoretically it should be in the center of the puzzle, but in practice, it might be slightly off-center. It's likely random. To avoid searching for a month, try to reveal as much of the puzzle as possible to at least see the edge of the cross.