Has this ever happened to you?
By the way, Catra is right.
Look, we have been multibooting for some time, and we are telling from our experiences. And you should do it in UEFI only, and I won’t suggest duel-booting in Legacy boot either, you can watch the video which I have, given in my first post, to know more about this.
And the mess you are talking isn’t really a mess, it is quite easy to solve:
You are already provided two guides, from which you can refer from.
And nothing is really going to happen to your disk, and it will never happen to your windows install , If it happens to Linux, you have already got the solutions in either of the above links, and anyway we are here only.
And nothing is really going to happen.
But this is important, you should learn what is BIOS and all its functions