The chances are you will get round on the day - most people do and its not the fastest of races by any means with lots of people under trained, walking etc.
Personally I would say 10 miles two weeks before would be the minimum target I would have aimed for to ensure that it wasnt too hard work on the day. If seven is your longest run so far then you have probably this week and next weekend to do another couple of longish runs.
We all have bad weeks. If your training up to then was solid then you should be fine.
No idea what pace you run at so no idea if 7.5m a KM is right or wrong for you.
But for me - stretching is one of those things you either do properly or not at all. I personally have adopted the not at all approach and so far it seems to have worked. Sometimes I will ease into a run with a slow mile or so and again a slowish mile at end but thats about it for me.
In terms of diet I think you dont need to go overboard for a half. Just the normal reasonably balanced diet. Giving up teh fags will probably help (although you always get the odd 40 a day sub 35 minute 10km runner to dispute that). Try and just eat sensibly - you are unlikely to need much additional fuel at this stage so dont overload.
Depends to a certain extent how fast you are. The closer to the front the more chance you have of an unimpeded run. The one year I did it (hated it) I ran pretty much my normal half marathon time but was in one of the very forward pens.
I think (nothing to base it on) if you are a sub 1:45 runner and probably sub 2 hour runner and are in right start pen (and near front of it) you shouldnt be held up much at all.