If you're going from platform 2 then the only train would be your train to King's Cross, so just get there before the train has arrived and people are getting off. You'd risk missing the train otherwise.
If getting the semi-fast service at xx02 and xx32 then the only train I can think of coming in would be the xx26 and xx56 (each likely to arrive at xx21 and xx51 give or take) in the other direction, so why aren't you waiting until everyone has left? You'd have loads of time. As in 5-6 minutes minimum.
You'd surely know the train is running at this point? If it was cancelled for some reason (train fault coming out of the yard or something) then would you abort your journey instead of getting another train, like perhaps a Moorgate stopper and changing at Finsbury Park?
You need to have written on the date BEFORE going through the barrier. Not asking to get let through and asking for a stamp (which I believe they'll insist on doing) and then writing on the date. The stamp can be washed off, so they'll be pretty upset if you are asking for a stamp and then offering to date it after. What if you don't? Obviously not suggesting you wouldn't, but once you cross the gateline you need to be in possession of a valid ticket, which it wouldn't be.
