I can't find this documented anywhere.

Is Priority in Firewall->Mangle, range 0-7?

And is the lower the priority number, the higher the priority given?

This page describe the priority numbers pretty well: https://wiki.mikrotik.com/wiki/Manual:WMM
Hope it helps.

Is Priority ... range 0-7?

To me it looks like any other label. Any string seems to be allowed. I already asked the forum about it:
viewtopic.php?f=13&t=142252&p=701712#p701712

Thank you