I have to say I haven't read all the posts but I wonder if by using toe clips (which I am scared to use cause I hear at least once you forget to unclip and crash-having thinning bones means I must be extra careful.) But if there is a lack of use on a muscle that keeps the toe on the pedal when you use them all the time. That could explain why even when you are careful and are consciously thinking of it it still happens. Muscle fatigue could explain the why. Perhaps keep one bike without so you keep the skill and muscles? Use it at least one day per week, that should be enough to not lose it?