Still only one responder? OK, I'll give this a shot.
Firstly a disclaimer: I'm a layman with absolutely no medical, physiotherapy or dynamic apnea knowledge. Have you spoken to a doctor and/or physiotherapist about this? It could be wise to have professional guidance. If you have to do something bizarre with your knees or ankles to produce the right foot position then doing hundreds of laps with fins could cause you other problems.
But if you insist on relying on us laymen... If you're physically able to position your feet correctly then practice / repetition should help. Perhaps start with very slow, gentle, precise movements while holding the side of the pool, to burn-in "muscle memory" of the required position before you start trying to build up strength. You may find that you can hold the correct position when there's very little water pressure against your feet, but when you push harder your feet flip inwards. If so that's great - now you can strengthen your muscles by gradually increasing force, and when your feet are strong enough to maintain perfect position then you can start swimming laps. As you suspect this may be good for your feet, but I doubt it would completely "fix" them.
But if it's physically impossible to maneuver your feet into the correct position then repetition won't achieve anything. Presumably it's not mere strength but the length of your tendons that's the problem, is that right? Have doctors/physiotherapists ever given you strengthening/flexibility exercises that you're supposed to do each day? If you've been neglecting your stretches then doing those would be a great starting point.
If your feet are so bad that you can't ever have a strong frogkick then I wonder.... is frogkick required for dynamic apnea, or can you experiment with underwater sidestroke? (The efficient version with a long glide between full-size strokes.) Foot position and ankle strength are less relevant in sidestroke, and underwater sidestroke's tricky but possible (and great for cruising beside critters).
Sidestroke is a faster stroke than breaststroke, but theoretically less efficient. However, I wonder whether the theoretical advantage of frogkick is lost in your case, because you have to expend so much physical and mental effort fighting your feet. Sidestroke isn't much less efficient - after all it's the basis for the so-called Combat Swim that US Navy Seals apparently consider the most efficient swimming stroke.
I hope something there helps. Let us know how you go.
P.S. I still reckon you should seriously consider getting professional input on this.
