Richard...You are correct to point out that there opportunities for 4 x 4 trips at a lower level in the Cairngorms.
Yesterday on my afternoon off-road bike ride, a circular one from the wildlife park to the ski road and then out via the Speyside Way section back to Kincraig, I came across a Land Rover full of aged folks on a trip, I guess, of the Rothiemurchus trails....all very friendly and we all shared a few words after they held back at a wide ford that I was in the middle of crossing.,,,and this very close to where you came out of Glen Einich towards the end of your walk to Braeriach.
I think what I was attempting to highlight regarding the Ski area proposal was that the driveable road in Coire Cas has existed for over fifty years. Whenever a ski area is developed inevitably some sort of 4x4 rough road, beyond the tarred surface up to the parking area, will happen as does happen in every ski area.
As far as I'm aware during the construction of the funicular railway no extra/new roads were cut on Cairngorm? Additionally the creep into other coires that the Grand Old Master wrote about did take place with ski uplift constructed into the Coire na Ciste. The two stage Chairlift has been removed as has the Aonaich T-bar uplift leaving the West Wall Poma tow for skiers in what to many folks is their favourite skiing section on Cairngorm. Skiers who ski down to the lower part of the ski road, when conditions of snow level allow, can return to the main area but the regular bus service from the lower parking area. There is still the poorly built old Café come ticket office building to be removed from close to where the Chairlift started.
I guess I am one of the few folks? still around who took their first steps onto Cairngorm when skiers walked from the 'Hayfield' with skis on their shoulders to reach the skiing in Coire Cas....not forgetting we school-age walkers camped on a rough bit of ground, now the modern Camping/Motorhome site at Glenmore.