Installing Windows can be quite a process, and it's easy to get confused about the best way to do it. I think it's smart of you to leave the partitions as they are and not stress about it. As you said, life's too short! As for the Windows 10 product key, have you tried looking on Reddit? There's a whole community there dedicated to helping people find product keys for various software, including Windows 10. I've personally found some great deals on there and I think it could be helpful for you too. Just search for "Windows 10 product key Reddit" and you should find some useful threads.