While I haven't progressed far enough in this run through to see if something like this happens, I wanted to share an idea. Maybe the MC tells Kevin about the cage at some point, like if he's wondering how complete the transformation is, you know, out of concern and totally not because he wants to know what he's dealing with down there in his fantasies! So you tell him about it and how it keeps the transformation from completing down there, maybe even show him if you're depraved enough, and some time on a different day, there's an incident that makes you take it off. Like, I don't know, you watch porn together, or your arousal is just too high, while hanging out and he notices how uncomfortable you are, so he suggests you take it off, and if you do, and go to the bathroom (because he keeps you well hydrated) without putting it back on, he'll switch it with an identical cage, but it doesn't have Jane's magic formula in it so it won't offer any protection against shrinkage! But you won't know, you'll keep wearing it to protect yourself, but it won't work! Of course when you inevitably start shrinking, you'd have to be able to talk to Jane about it, and she should just be too busy to help, or brush you off, or tell you to *relax*, and you're doomed to shrink away!
Oh! Or instead of those reactions from Jane, since he swapped it out, HE now has the only working key! So Jane can't really help, and he could either keep it secret as you slowly finish transforming, or you have to figure out who has your key, or learn to pick the lock, or he could reveal it and there could be a whole keyholder relationship! (yay! more work! lol)