"but I feel footlocks that twist are too dangerous for beginners, simply because of the risk of debilatating injury from the knee that can occur for no reason."
I disagree. The lineage that I came up under teaches all footlocks (including heel hooks), starting at white belt, and no one ever got their knees screwed up. The key is good instruction. We were taught to secure the position, take the slack out, then "pose for the camera". And we were taught that if someone footlocks you (especially the heel hook), and you haven't learned how to escape safely, that you better tap. If you do something stupid, like try to twist out, and you hurt yourself, it's your fault. I trained for years under that methodology and no one had any problem with it, and no one got hurt.
Then, later I moved and ended up at an academy that doesn't teach footlocks until purple. What I discovered is that we had purple belt and above level players with white belt level footlock game. It allowed me to footlock guys with impunity. I started being known as the footlock guy, even though I don't consider footlocks to be my specialty.
The footlock game is just like any other game in BJJ in that it takes a long time to master, and in that you have to play it as a position with control. If you wait until purple belt to head down that road you'll have a beginner leg game as an otherwise advanced player. All knowledge should be shared from day one if the goal is to be a complete fighter.