PitR's
I congratulate you on making the early train situation sound like a nightmare but you've cleverly been selective in making it seem worse than it is. Perhaps I can be allowed to put the facts a little more accurately.
Nobody it their right mind will take the 0459 departure which, as you say, takes 93 minutes and involves several changes of train arriving SEN at 0631. Why would they not do this? Simply because the 0528 departure arrives SEN one minute later than that at 0632, takes 64 minutes and is a direct service! The 0555 departure takes 52 minutes and arrives SEN at 0647.
So in reality you could reasonably rely on arriving by 0647 even if one train was cancelled, which to my mind means catching a flight with an ETD not earlier than 0730, but ideally not before 0745.
Is this satisfactory? No, it patently isn't and Stobarts seem to be finding it extremely difficult to persuade Network Rail that they should open up the line even half an hour earlier to allow an 0600 train arrival at SEN. It must to a degree hurt the AMS, BFS & DUB figures which all have departures before that time.
As far as the roads are concerned, the M25 is 20 minutes drive (plus 200m walk from car park to check-in desk) if you're catching one of the first wave of departures and it's all dual carriageway except the last 1 km. You are correct in saying that you drive through residenial areas to get to SEN; exactly as to do (although even more so) when driving to LTN from the east. Driving times to London are, according to Google, 57 minutes to Docklands, 60 to Bishopsgate, 65 to Chancery Lane and 73 to Marble Arch. There is also a choice of two routes; via the A127 or the A13 should one be blocked by an accident. I don't quite see why anyone would specifically want to get onto the A406 North Circular as part of their journey when they would use the M25 until they reached the appropriate exit.
As you quite rightly say, it all depends on where you live/where your UK destination is as to which LON airport will suit you individually but I would suggest that many more than just "locals" find it the most convenient and quickest departure/arrival route.
Lastly, take a look at a map and you'll see SEN isn't on the North Sea and if IATA (rather than just the airport authority) decide it's a London metropolitan airport then that's good enough for me and probably most travellers.