thanks for your reply but I'm still in the dark.
what I really need to know is if I flash a bootloader from a different OS version, without TWRP etc, ie a straight swap of bootloaders, will the device still work normally? I don't want to try anything until I know I will be able to get out of trouble and have a functioning device.
I appreciate your advice to put a ROM and GApps on the sdcard, but what ROM? Stock? Do you mean the bin file or 3 images?
Or do you mean CM11 or similar?
Installing CM11 is my goal, but having run into trouble the first time after installing TWRP I want to reduce risks as much as possible. My intention is to install TWRP first, then make a backup of the stock ROM so I can go back to that if needbe, then to install CM11 and give that a go. The only thing stopping me is that the first time I tried, after installing TWRP I couldn't boot up the stock ROM, and I don't know why it happened.
Would I be better off downgrading to 7.4.6/7.4.8 before doing anything else? I assume I'll lose all my apps and settings if I do that?
In any case, I'd like to verify the checksums of any file I download to be on the safe side, but haven't seen a comprehensive list of files/images and checksums. Where can I get that info?