Regarding your edit, that was true in the past, but Apple fixed it in iOS 8.2. Now the system makes sure to write the failure to nonvolatile storage before reporting it. Phones running older OSes could be exploited this way. There are even boxes you can buy that will do it for you automatically. You set it up to try a passcode, then cut the power on failure. But they don't work anymore, and this phone's OS was too new for it.
There's a mechanism that wipes the device after too many incorrect code attempts. Disabling this mechanism was the entire point of the FBI/Apple lawsuit.
Why would you be pretty sure of that? That would be totally braindead if a simple reboot bypassed the limit. It doesn't. There was a flaw in an earlier iOS version where you could reboot the phone a few milliseconds after an incorrect PIN, before the phone recorded the failure, but that was fixed some time ago.