Let me try and address your concerns
Lack of cruise on the Duke I agree is a pain, the new 790 adventure bike does have cruise, The longest rideout I have done is 8 hours only stopping for fuel and a leak.I try and avoid the big open roads when riding he 790 so the lack of cruise put up with. I use my 1290R which does have cruise.
Engine buzzing vibrations, this is the smoothest twin I have ever ridden and I have never experienced any vibration related discomfort on this machine so I would so do not worry.
Riding at 70-85mph, well any naked bike as we know at these speeds for long distances can become tiring with wind blast. their are screens available, but if this is your major again I would say consider the adventure version.
The +20mm ergo seat double the riding hours I can do in comfort, it reduced my knee ache and bum burn significantly. a must for long hours in the saddle