My preference would be to be able to boot from the DVD so that I can re-partition the drive for a true wipe/clean install experience, but I'm not sure this will let me do that.

Step 2: Power on your Lenovo computer and make the USB drive to boot first. I haven't gone through with it, but wanted to mention it in case anyone's done an OS install like that before. If you have the same problem as me, that is the fan dont work because you. It even warned me that if I install Win10 in that way, that I wouldn't be able to revert back to Win8.1 (presumably because it would wipe out the Windows.old folder where it's stored). I then tried to just launch the Setup.exe file that's on the DVD, without literally booting from the DVD, and that seemed to successfully launch the setup/install process. I even tried the Advanced Startup Options inside Win10's Settings, where it gave me the same list of choices that I saw in the BIOS, I selected the DVD drive, but, same behavior. Unfortunately that didn't seem to help, as it still behaved the same way, the DVD spins up like it's being read, but nothing happens and it ends up booting into the HDD-installed Win10. But I changed the boot process from 'UEFI' to 'Legacy OS Support', and that seemed to then set Secure Boot to disabled.

Well I found the Secure Boot option but the BIOS wouldn't let me arrow down to it to select/change it directly.

Lenovo external dvd drive not working