a - I agree, I would pay for thicker/heavier iphone with 2x battery life. However this will fracture Apple's product lineup, thus economically unfeasible.
b - screen on iphone 5S is already very strong.
c - After lots of thinking I agree with Apple. Current form factor is just right for communication device [1]. If you need bigger screen, chances are you do not want a phone - you want a tablet.
[1] Iphone user here, take this with a grain of salt. Over 3 models of iphone I never had a case. Phone is small enough for a proper grip with one hand while using thumb to control it.
Anecdotal experience: I know 12 people with phones that have large screens. 7 of them had screens cracked due to dropping the phone. Every single one has a case (People who dropped their phones got a case after). What is the point of getting super light and thin phone just to put bulky case on it?