I'd be fine if we start the season with a back 3 of Casey, Mugabi and Blaney. I'd argue positional awareness is very much something you can coach, but it's easier if you play in the same position every week rather than the full back, centre back, wing back, striker positions we've asked Bevis to play in his rare starting appearances.
I agree Lamie and Mugabi in the same 3 doesn't seem to work.