Like the second option best, but not totally happy with it.
I think I would either go with a solid corner or do a 4 patch with the two small white squares leading the eye into the center.
or...maybe use the diamondy square in a square block for your cornerstone?
The small white square just doesn't look properly proportioned to everything else, although I see your reason for trying to echo the 'arrow' look of the main blocks.