There are no chances with the boxes :), is always 100% if you get one of the conditions above
Coulda sworn I read somewhere that you had a chance to get a box when you win as long as you had boxes remaining.
You queue w/ someone and he gets an S, AND you didn't get a box in this season with this champion AND if you have remaining boxes to earn.
This basically tells me to stop playing with my friends if I don't have any more boxes to earn.
I really don't like the way they implemented Hextech. It's bad enough you can only earn so many boxes every few days, but to make it so you can only earn one box per champion? No, I don't actually want free things if I have to carefully monitor my champion pool.