As for will we have to buy it again- I wouldn't do all that work for free, would you?
Of curse they'll be more features in the PC version- all the stuff Craig envisions adding to the game, but the 360 holds him back, he'll be able to with the PC (and One).
As for the rest of your post- this isn't minecraft. As usual, there might be something better!
First Off, I don't think you have a right to speak for Craig about his onw game.
Now in all seriousness:
Craig may not want to have diffrent features on PC. He may want the games to be the same and just make it for the variety in systems and benifit of gamers of any platform.
Just because this isn't Minecraft donsn't mean there can't be some similar aspects to the game. Maybe(as MCEdit was not made by the makers of Minecraft) somone will go and do their own world editing system for TM:F.
Craig also hasto worry about(to keep all PC gamers happy) how not all people have computers foged by God. So Low spec computers will still be a problem with adding too many RAM resourceful features
Craig may feel that the money people spent buying it on xbox is satifyong enough and make it free on Steam.(Of Course for the people who bought it.)
Again these are ALL Craig's choices so we can't control what happens. These are just my OPs on the matter. It could go either way.