The Google+ backup photos won't automatically sync back to the device so won't be picked up by the Gallery App after a factory reset. You will see them in the Photos App (actually a sub-component of G+) as long as you have a data connection. You can download them back to the device from the Photos app as-well if you really want a copy sat on the phone.
Hopefully you've already checked this out but G+ photo backup has options to backup full size photos or to resize them to longest side 2048px (with the former option eating into your G Drive storage). Double check what settings you used if you want to ensure you retain the photos at original quality.
Personally I wouldn't use the inbuilt option as my standard. My photos live on an always on Media Server at home, which backed up to an external drive at home, G Drive (via the PC app) and G+(via Picassa). I just manually copy/paste photos over to the Media Server (which is a SMB share) when I'm on my home wifi via ES Explorer
Enabling encryption won't impact on OTA compatibility. Rooting could easily cause problems, particularly if you make changes / install stuff that changes the system partition after rooting. But it's easily fixed simply by reflashing a stock image, updating and then taking the necessary steps to re-root etc (it seems likely rooting is going to be a bit more of a PITA under lollipop).
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