If you reflash the same ROM there is basically a guarantee that it’s 1 again. Once you boot it for the first time you will end up with a 0 (at least with stock FPOS and CalyxOS).
That’s why you shouldn’t boot into Android and lock it afterwards, that’s how you end up with a 
No, the install script will set the slot, don’t mess with it if you don’t have a good reason. Switching slots won’t bring back get_unlock_ability.
If your system is unbootable, your bootloader is locked and you have lost get_unlock_ability, then there is no way for you to switch slots. Changing slots only works with an unlocked bootloader.