Reinstall original OS without rooting (Oneplus One, CM11S) I've a Oneplus One running the original OS that came with it (CM11S). Accidentally replied 'Yes' to  a google security prompt which identified the Settings app as harmful, and it was deleted. 

As far as I can determine the only way of retrieving it is to reflash the ROM with the original OS, which I've downloaded a copy of. I've backed up most of the stuff on the phone, but if possible I'd still rather not go through the entire process of rooting it/unlocking the bootloader, etc. I haven't done it before and I'd rather not go through the hassle of wiping the phone and restoring everything afterwards.

If I understand the process properly, the point of unlocking the bootloader is to allow custom ROMs to be flashed onto the phone. I'm wondering if I can bypass this step if I'm only interested in re-flashing the original ROM. (Although I've a feeling that might still end up being seen as a security risk so not possible) 

Or if there's any other way of getting the Settings app back, I'd appreciate suggestions
You can use adb and sideload the zip of the OS from OnePlus using the stock recovery. Here is a link to discussion with some tools to help you flash the stock rom easily.