There still seems to be more overlap (more poly folks who permit one-night stands in swinger-ish manner than monogamous folks who permit it). Do you find this not to be the case?

Hmmm, I'm not entirely sure. In my social circle far more monoamorous people #PRACTICE# one night stands (in a swingerish manner) than polyamorous people. The polyamorous people may #allow# it; but when you can date whoever you want, and aren't forced to limit it to a one-night stand, why would you limit it?

My social circle is, however, distinctly atypical, and so cannot really be construed as evidence of much.

Most poly partnerships keep their sexuality limited to the 3 or 4 or 6 of them, and would look down on a partner having sex with people they didn't intend to add to the long-term group?

Groups suggest a closed loop, which is uncommon. However many poly people I know are uninterested in having sex with anyone who they don't feel a romantic bond with, simply because they have far more satisfying alternatives available.

How common is it in your experience for the polyamorous to have non-sexual romantic partners?

Maybe 10%, or so. Not massively common, but certainly not unheard of. Far more would be open to non-sexual romance, just haven't had one.