I feel like being able to move the pivot point with RMB is too convoluted. Maybe moving with WASD+QE moves the pivot point, LMB checks if tile is a mine, RMB flags the tile, Scrolling zooms into or out from the pivot point, and middle mouse button pans around the pivot point. I think on most normal shapes just being able to pan around should be enough, being able to move the pivot would only really be necessary on custom boards.