The first problem is entering the pass against butterfly guard, it's so much work, the guy as grips, hooks, you don't have a good scoop grip on the leg and you are most of the time too low to put pressure
I like to get to half guard / headquarters and work on smash pass and kneeslides. With all that movement, a good scoop grip on the leg and some practice you will enter the over underpass a lot deeper straight away and make your life a lot easier.
It's also a great pass right after X guard technical standup sweep and deep half sweep because the leg is already over the shoulder
For the straigthening of the leg with your arm, it works well, but the Kimura threat is real. But I do it a lot without the hand grip, I focus a lot more on the movement and the pressure I put with my knees
The game changer for me was tweeking my knees inward, it's from Bernado Faria's DVD
So if you trap the leg on your left side you will tripod, pinch your knees together and point your knee caps sligthly to the right. Then you start your movement outward to straigthen the leg and you come back to make the back step / windshield wiper easier.
When you get the right angle on that tweek you will put a lot more pressure with your shoulder on the hips, and it helps to unbend the knee, like a side pressure