1. cnMaestro currently does not support PTP800. It currently only supports PTP450 variants, and there's been talk about support for PTP6XX. I haven't heard any talk about adding 800 or the newer 820. Apparently, the 800 and 820 will not be supported, see THIS thread. There's been talk about adding SNMP monitoring for radios not natively supported by cnMaestro (for instance, this would allow some basic connectivity to PMP100 legacy radios).
2. cnMaestro is evolving rapidly and more features are being added all the time. There's too much stuff to go over here. I'd start on the cnMaestro page HERE, and then watch some of the latest videos that show a road map of what's to come. I think the biggest difference is that cnMaestro can be used in a cloud scenario or on a dedicated platform. I believe the future of Cambium's management offerings lies with cnMaestro with development for WM drying up and eventually it disappearing.
3. At this point cnMaestro is free to use (cloud or dedicated), but there's been talk of adding premium features.