Summary: Solution found - hardware issue
Introduction: Hi i've been following this thread for the longest time as i myself have encountered this issue around a week after updating to iOS9 around early November.
I tried all the suggested steps, hard reset, restore to factory settings, toggle battery saver mode - and even tried squeezing over the camera as suggested.
As many others have identified the symptoms- black rear camera screen, yellow alert icon at the top left; when tapped on says flash needs to cool down..; and a non functioning greyed-out flashlight from the swipe up panel.
Alas i decided to give opening it up a try.
Disclaimer: Be warned- will void your warranty if done outside of apple's supervision- only proceed if you are out of the 2 year period- if not do send it back to them
Procedure:
I'll keep it brief and non-technical as i myself have close to zero training on operating on smartphones.
If you've ever seen a teardown, you'll know that once the bottom 2 penta-screws (neologism most probably) are removed, you will 1st have to remove a ribbon on the inferior (lower) part of the phone.
The screen swivels open with an attachment at the superior (top) part of the phone, with a further 3 ribbons supposedly attached snugly.
Findings:
One of the 3 ribbons was either loosely detached/attached- and simply put i believe the ribbon belonging to the camera module was loose.
I basically took all 3 ribbons off, reattached them and piece the screen back in to place (not forgetting the last ribbon i.e the one at the inferior pole of the phone)
End Result:
The camera, both rear and front facing- worked the charm, as did the flash. Meddling around pessimistically with it the past day- still no signs of return of those dreaded symptoms.
Discussion:
I was initially sold into the idea that this was definitely a software issue, as many have pointed out that it only happened after updating to iOS 9.
However, after scouring through many other threads, i noticed this "flash cooling down- blank rear camera" problem seemed to affect a lot of other people whenever they upgraded to a newer operating system.
I theorise that it could be because we always try to find a triggering event to put lay blame onto an adverse outcome.
I noticed also that many here encountering this problem were well past their 2 year mark, or were approaching it- which lends to the conspiracy that Apple would try to kill our phones through the software updates and so on. Though admittedly, some of you here reported the problem even with uber new 6ers of 6Sers.
Screws become loose, and connectors dislodge across all appliances smart or otherwise, over time.
Could this be software related? Maybe. I think someone mentioned this earlier- sorry i can't remember who
Could it be possible that the newer software was made to be a little more sensitive/insensitive to the loose connectors, such that my previously hanging-by-a-thread 2nd ribbon was detectable before the update, but the newer iOS was less forgiving towards loose connections?
Conclusion:
Again be reminded that the root cause is hardware in origin, which is best fixed by the guys at Apple.
I cannot guarantee that this will work for you, but i sure am glad that it did for mine