Re: Irimi Nage
I don't think you should worry too much about controlling uke when they are dropping towards the mat. Honestly, getting behind somebody and dumping them on their butt is a good technique. The practice then becomes, staying connected with them as they get back up, and finding the sweet spot where it is easy to throw them the second time.
I was once taught that a combative application of irimi nage was to get their body torquing one way, and their head the other - snap. That's a hard technique to practice though.
One of our head instructors, Don Moock. teaches getting uke's shoulder stacked directly over their OPPOSITE hip, with their head over the hip. He lays his arm over uke's shoulder to make the connection and unbalance, rather than grabbing the neck. It seems like you can get most ukes into this posture before they feel terribly off-balance and try to get it back.