Unity Free and iOS Basic Development

I've been looking for game engines that are cross platform between PC/Mac/iOS and Unity seems to be a good engine for this.

I am planning to develop an iPad game using the Unity free and iOS basic as from what I've read this is sufficient for what I need.

My questions are;

1) do I develop the complete PC/Mac version first in Unity free, then when I'm ready to do the iPad version continue to license the iOS basic?

2) if I do the complete game in Unity free first then later license the iOS basic is it difficult to port into iOS basic?

3) if I develop the PC/Mac game it will be on a PC however when I do the iOS basic build that will be on a Mac. Are there issues with getting the code from Unity free from the PC build to work with the iOS basic version which will be on the Mac? Or should I just develop the PC/Mac version of the game on the Mac in the first place? Or doesn't it matter?

4) should I make the game in the iOS basic first? I do want the game across all 3 platforms but ideally it is to be on the iPad.

Thanks