Are the engine thermostat and shroud flapper assembly considered integral to providing heat to the cabin/ defrost systems, or are they only for directing air over the oil cooler??

No those are only for the warm-up of the engine.

The latter, which does matter.

well I have a propane heater with a small bottle, and a thermostat, the 914 is warm and toasty, 0 warmup... I also have an electric heater un GRNMNEE instant warm....I also have a big ass alternator,
My heater is for bobcats and other contruction vehcles, mine is 30 amp and 50 amp draw.

http://dcthermal.com/catalog/sa12-2000

It a square box 6”x 5” X 4” under the dash facing towards the back.. I have made a splitter box to run defrost and interior heat but not installed.

How well does it warm up the cab?

I'm betting you have a warmer car. Who cares in a 914? I found en electric solution for defrost that worked but it was fortified by rainx.

considering its dry warm heat from the second you turn it on , it works very well, need to be ducted to defrost..

Very interested. I am not wishing to go with a gas BN4 if I dont have to in the bus, but it is too cold to drive without defrost and some heat. How well do you think one of the SA12-2000 12 Volt Brushless Cab Heaters would put out for the front cabin of the bus? Is it appreciable? Ducting for my app would be pretty easy as all the vents are right there, like a wall in front of the driver.