I'm not in exactly the same position but I do have a pair of 11s that are pretty old now. I've been thinking of buying some 12s but after reading your post I'm not so sure. I certainly don't want anything heavier or bulkier. Maybe another pair of 11s is the answer..?
I had a disappointing run at London this year and have also found it harder to recover from than previously. As far as I know I didn't have a bug. Perhaps it's got something to do with the amount of effort needed if you're not having a good day. Whereas if everything's coming together nicely, maybe that's less stressful on your body (and your mind).
I can't see why you would start walking when the finish is in sight – surely that's the best bit of any race?! My trouble is more at the beginning when I'm doubting I can do this and thinking of all the effort that's to come.
They look very new (well, clean) in the photo. That happened to my Inspire 10s too, but only after quite a lot of wear. I think Mizuno were going a bit far trying to make them lightweight and they seem to have rectified it in the later version/s.