Before I start answering more questions, I just wanna suggest you have a look at the
iOS Developer Program pages from Apple, as well as their
Support site. Lots of good info there.
Basically, you get Xcode as well as access to the restricted parts of the developer site and iTunes Connect. No, you do not get an icon maker.
I'm not overly familiar with the Metal framework but my understanding is that as long as the device has an A7 or better, you should be good to go. Perhaps this document can provide a better answer:
Metal Programming Guide
I don't understand this question.