Write the last 2 weeks off - There's no catching up at this stage unfortunately but seeing as your training has gone well you won't have lost much in fitness/readiness.
For me, I've only ever done a 2 week taper and it has worked perfectly well but everyone's different. My weekly mileage is pretty high and I'd find a 3 week taper just frustrating. You've certainly still got time to get a couple of long runs in but the last thing you want is to be standing on the start line having tired legs. If you feel you'll need a 3 week taper to recover then that's what you must do.
No need to stress, you'll still be in good shape and it's better to get a cold/chest infection now rather than nearer race day.
Hit your 10:16's straight away ensuring you don't get carried away with the excitement of the start. If at mile 20 you're feeling strong (relatively speaking - you're bound to feel knackered!) have a go at picking up the pace a little.
Personally I wouldn't try and bank time for a later fade as all you're doing is encouraging a miserable last 6 miles. Rather bank some energy for a strong finish.
Good luck. Like Keith say's we all run our races differently so there's no right or wrong answer, just preferences.
Big Bertie - Yes. I did it a few years ago. Apply for the GFA and if you manage to achieve a Championship time then give VLM a call straight away. Provided they haven't allocated and finalised the numbers (think this happens in January) they'll put you in the champs start.
Can't see why not although any training you do between now and then will be limited. You need a couple of weeks rest now and then a decent taper before the next so you've really only got a 2 week window to get any key runs in.
I ran a half marathon pb 2 weeks after running the New York marathon so it's achievable.
I also think you need to work out what went wrong at London - cramping at 16 seems very premature. Were all of your long training runs run at slower than marathon pace?