If a given motherboard is capable of booting an O.S. from one of it's M.2 slots, does that mean it can also boot from a M.2 SSD if it's installed in a regular PCI-E slot via one of those adapter cards?
Yes. There is really nothing special about an M.2 slot, it's pretty much a PCIe slot in a different form factor.
With my previous system (what is now my "backup" system in my signature), the X99 motherboard I got did not have an M.2 slot. I went for a long time booting via a PCIe adapter card that had an M.2 slot on it. The card did not have any electronics on it, it simply routed the traces from the M.2 slot to the PCIe slot.