Re: Uke spinning out of shihonage
As they're spinning out, turn in tightly and go for an ude-garami. Don't hurt them. You can always stop a bit before the takedown -- the "tapping" point. Whatever you do, I advise you not to get all crazy about it. It's just practice/drilling. If you don't have enough live practice, then do some freestyle. Don't try to mix forms, drilling, and sparring.
As for avoiding it happening in the first place, make the duck-under lower and the hold tighter. That dramatically reduces the amount of space they have to spin out with. Really pour a lot of energy into the drive-forward thrusting movement and take it to a very low stance, and don't rise back up as you turn and cut.
Another idea which I'm not as sure about: if they're spinning out, I think they're turning their back to you. You could potentially just let go of the hand and do ura-nage.