The pain thing is entirely down to your description. If you say I place you under this and you feel lots of pain, chances are they'll dispute it. If you detail it... a sudden flash freeze shoots through your body before settling behind your eyes, then, as your eyes begin to freeze over, the illusionary flash freeze forces your brain to react, numbing the nerves around your eyes and causing sharp pains blah blah.
Basically, you want to make the pain seem like a natural effect of the body, under the impression it is freezing from the inside, rather than an effect of the genjutsu.
As for the dormant thing, you use the handseal, next time they take a step they're freezing. It doesn't matter how long after your original post it happens, as long as they haven't released it then it'll activate itself. You just have to post the technique again for sportsmanship
Lili always told me to leave the genjutsu as vague as possible, it gives you more control when it comes to you using it.