Thanks for the feedback. I can see the argument that polyphasic sleep is anything more than one monophasic block of sleep. In practice though, I'd say most people differentiate between a biphasic schedule and a polyphasic one.
Napping is absolutely sustainable. Trying to reduce total sleep time by sleeping in small bursts throughout the day and night does not appear to be sustainable over long periods.
Good point! I actually mention Ekirch in a separate post. I'd call what he's talking about "biphasic," because people in that time slept in two phases. I personally prefer biphasic to involve an afternoon nap.
I know there's lots of science supporting what you say about sleep being essential for consolidating memory.