Yes you don't need the newest mac to port it, it just must be as powerful as the iphone

Unity is easy-to-use for iPhone games. Though it also depends on what kind of game you want to create. For 2D games, i would probably suggest iTGB from Garagegames but for 3D games, Unity is better.

Since i want to make 2D and 3D games, i got Unity. The scripting is super easy too.

Dunno about Shiva, probably a good solution too if you have enough programing knowledge.