I am just curious because on some other aircraft with an AF installed, there
was no install option, nor any choice to fly with out it being operational, that
as you touched upon, the reason was the unfeathered profile was so disastrous
that it was actually a negative climb rate at VMCA.
That doesn't make sense. Plenty (most?) twins have negative climb at Vmca unfeathered. Aren't there different certification rules being muddled here - ie. how Vmca is derived for certification purposes and the requirement for autofeather, and how SE climb requirements are calculated (clean/feathered/Vyse/Bank into live etc)
My understanding is that on certain KA models, Beech (or Raisbeck) wanted to take advantage of the lower Vmca available from AF and asked the FAA to accept a Vmca when feathered (rather than the windmilling standard) if AF was fitted. That made the AF a required item both for certification and operationally (since they didn't publish "No AF" performance). However there were 4 blade KA models that didn't require AF, had different low pitch mechanisms I think.
Don't take my word for it, it's in Tom Clements' King Air book, from p205.