1. This is with systemd-boot, which I switched to because it's easier to use a unified kernel image with, but it should work just fine with grub as well. The last step will sign everything that needs to be signed, including grub and the kernel images.
2. You only need to trigger a re-sign if you update grub using grub-install. If you just change the grub config, you don't need to re-sign it because the config is loaded once the signed grub is already booted. This is another reason why I went with systemd-boot and unified kernel images, because I work with sensitive data and maybe I'm a bit too paranoid, and don't want anyone to be able to tamper with my boot in any way. This is also possible with grub and using an encrypted boot partition, but systemd + UKI + full system encryption was just easier. If you're not worried about evil maid attacks and just want secure boot, grub will work with no additional setup.
3. No issues with the pacman hook, it triggers every time there's a kernel update or nvidia update, and since I'm using mkinitcpio and UKI, the signing is usually already done by mkinitcpio before the pacman hook is ran, so the pacman hook doesn't really ever do anything. It's all done in the mkinitcpio hook.

As for bricking your motherboard, this only happens if your motherboard or any other component uses the microsoft vendor keys as part of the boot sequence, and it's only really a hard brick if it's your motherboard that uses it. If it's any other component, you can remove it and readd the microsoft keys and it'll work again when you add the component back.

And the key part here is replacing the platform keys. If you just always use the -m flag on sbctl enroll-keys, you'll enroll both your own keys and microsoft's, meaning no replacing necessary. If you always use -m, there's no real risk really, because you'll always add the microsoft keys that your hardware might need. Plus, if you're dual booting with windows, you need the -m to have windows secure boot work, anyway.

If you're extra paranoid, you can also add the -f option which should also include all the keys that your motherboard comes with by default, if it contains more than just microsoft's keys, but this shouldn't really be necessary.