If the foot becomes too much dorsiflexed then Achilles tendon might be ruptured. I am not sure for the first one.
That's the answer I found. It usually occurs in the sprinters at the beginning of the race due to sudden strain on the tendon. In case of normal people there can be some dystrophy but I'm not sure. (By the way why can't I reply to you directly??)
For your first question it's probably due to the fact that the medial head got greater attachment proximally and longer insertion distally than the lateral head. Hope this will help (nice question though).