This is a great thread, thanks for sharing. I saw the Mini-PCIe to M.2 adapters for NVMe drives on eBay, and I was like "That's not really possible, is it?"
Do you have any NVMe benchmark speeds you could share with us? Did you ever find out if the slot is 1.0 or 2.0?
BTW I've used Clover to boot from NVMe in a desktop PCIe slot w/ adapter, in case you're interested: https://www.win-raid.com/t2375f50-Guide-NVMe-boot-without-modding-your-UEFI-BIOS-Clover-EFI-bootloader-method.html
Usually Clover is used for making hackintoshes, so most guides are Mac-centric, but you can make a Clover boot disk using Boot Disk Utility (link in guide). Booting using an NVMe requires a usb flash drive (or other drive) for bootloader, but you can take it out until you're ready to reboot/turn on the computer again... Or you could put it on the EFI partition of the PATA drive.