Yup, agreed. I have a couple of clone boards and both report as an unknown board. (although not as a Z-stick!). The Leonardo R3 by contrast correctly identifies itself as a Leonardo.
Z-stick has nothing to do with Arduino ;)
Since you could program the chip via the Mega, that suggests you can communicate with it via the TX0/RX0 pins which suggests it is working, so one wonders whether the issue is with the UART?
Not quite; burning a bootloader is done via ICSP. You however have a point that @fxguy526 can possibly use an external USB-to-TTL converter to keep on using the Uno.
So how do I troubleshoot the USB on the Uno?
No idea how to debug the USB. Bad cable (it's probably the same as you used for the Mega, so not likely), not properly inserted, bad socket on the Uno, some faulty adapter that is need when using Macs (not sure if you need to, I'm not a Mac user), ...
Do you have an option test on a Windows machine? You only have to check what happens in device manager, no installations needed unless you want to.
Once it's recognised and you can use it, we can look at the PU 'issue'; I have no idea, you might have to install a different so-called core.