5. upsc, upsstats, and the other clients say access denied. The device communication port (serial, USB or network) permissions are fine, so what gives?

In this case, "access denied" means the access to upsd(8), not the device communication port. You’re being denied since the system has no permission to speak to upsd according to its access controls.

There can be due to various reasons. To fix it, check:

Refer to the upsd(8) and upsd.conf(5) man pages for more information.