First order of business was to wipe the HDD and install Windows 7. This was done both to avoid the many downsides of Windows 10 and to get rid of the strange (but hopefully not malicious) configuration that was on the laptop when I arrived, with 3 separate accounts including one password-protected one under the name of the shop I bought the laptop from.
Problem: Windows 7 Pro OA key not working during install (unresolved) Installing Windows 7 Pro OA using the key under the battery compartment didn't work, likely because of differences between the installation image I was using and the official Lenovo one. Instead I used a separate Win 7 installation and key that I had on hand, and had no real issues.
Reaching BIOS / Changing boot order This is simple, just hit Enter during the splash screen immediately after powering on, and then on the next screen quickly hit either F1 for BIOS or F12 for a boot menu.