Mobile chips are usually smaller than desktop chips. That alone is enough to increase the yield significantly.
Low-end iPads and AppleTVs where either frequency or power consumption isn't as important, respectively.
Apple's history is mostly using older designs for the lower end devices. They are completely different chips, not just a lower frequency version of the same part.
Sure, but things can change?
Exactly.
The answer is nothing. If it isn't already, TSMC will eventually pass the cost onto Apple.