FFS …. the HTML tag is “form”

I can answer my own question. Perhaps it will help some people.
PayPal’s website payments pro is NOT supported by Nationbuilder.
PayPal website payments standard uses the PayPal express checkout (which transfers users off your site and onto PayPal to complete payment).
Express checkout allows users to make monthly recurring donations (but forces them to create a PayPal account).
PayFlow Pro keeps folks on your website through their payment step but is intended for higher-volume accounts. The fees are also not attractive.
We ended up using eWay rapid AND PayPal accounts.
PayPal for donor-initiated recurring payments and eWay for lower fee payments for everything else. Also some people just prefer to use their PayPal account to pay for membership etc.

It’s possible to use eWay’s portal to do recurring payments, but Nationbuilder doesn’t support even noting these office-initiated recurring payments against the donor’s profile, let alone using eWay’s API to submit donor-initiated recurring donations.

Overall, a pretty messy result for us requiring significant administration behind the scenes to make it all work.