In fact magit could probably do some of this with a hook on reordering commits which would not require more git metadata.
I'm not sure I care about any of this though as if I do rebase, reorder and then hit a conflict I can't resolve I can always abort the rebase.
My original idea was to somehow show the actual conflicts, but then I figured this gets the desired 80% for 2% of the effort...
In fact magit could probably do some of this with a hook on reordering commits which would not require more git metadata.
I'm not sure I care about any of this though as if I do rebase, reorder and then hit a conflict I can't resolve I can always abort the rebase.