Martin, Not to state the obvious, just an FYI. You mentioned 2 hard shell boats on the outside of an Otter.

I've been in an Otter with 3 Old Town Discos inside, with 6 passengers, plus the 2 pilots. None of the boats needed their thwarts or seats removed to nest as I recall.

We were full, but OK. Even better, everyone using bag boats like  DD's would allow for more passengers  :thumbup
I've gone that way too and you can easily do 8 passengers and gear. Better for adjusting the load in the plane.

The guides/pilots were careful the load wasn't tail heavy when we used hard shell boats..

Sounds like fun.