I think you can takeback moves after you've hit submit. I assume that's until your opponent moves.
Resolved: takebacks
Looks like you can take back any number of moves at any time in those games: http://support.chess.com/Knowledgebase/Article/View/17/3/can-i-take-back-a-bad-move.
Of course, such games are unrated.

I don't know about inhibiting learning. Once you've realise what you've done you've learnt something... supposedly.

Yeah but you remember something better if it costs you more. I think it will make you play with less care and less consideration of your opponents threats if you can try out moves and see what happens rather than calculate the continuations for yourself.
It's better for you to spend some time and realise your opponent has a potential knight fork than to play a move and once they respond with that just take it back. You need to learn to be able to predict their moves and play the best moves even against best responses.

There wouldn't be a game if you used takeback all the time nobody would get anywhere. You know what a big deal a loss is whether or not you use takeback. What you say is true, you do need to spend time and think about your moves it's all part of the game and i'll agree that it takes something away from it. The challenge maybe or achivement. Thus why i won't be using it.
What is the difference in a takeback challenge? Without wanting to sound stupid.