I think it would be nice to have separate color configs for flexible block placement adjacent, flexible block placement offset, and flexible block placement rotation overlays. For example, "offset" could be blue, and "rotation" could be green, and "adjacent" could be red.

I'd second with the addition of a letter in the center (O, R, A).

And possibly a similar marker to show what 'fast placement' mode one is in (like face fast placement, skip destroy), etc.