The kotegaeshi you are referring to is one of my utmost favourite techniques. My execution however is slightly different.
Instead of placing the arm across my chest I tend to break Uke's posture downwards a bit and place the arm across my hips. This has the same levering effect that you are referring to, but is extremely stable as long as you move from your hips and not your shoulders.
I have been known to utilise the "clamp into armpit" version myself, but again, instead of placing it into the armpit where it is easy to unbalance Tori, I instead drop my elbow lightly over the arm to secure it to the side of my hip. This gives the same effect of levering the arm against my body without the increased possibility of it being countered.
From my experience resistance to this technique is often futile when performed correctly. It's effect as kuzushi becomes really effective as resistance causes levering against Uke's elbow, causing a reaction that just helps the technique. It's one of my mainstays for resistance tanto randori.