Typically, these little USB mixers send the same output to the USB and analog outputs. So you might need to monitor the backing track through your soundcard as your playback device and record guitar-only through the mixer.
If you monitor the guitar through the computer there will be some latency (delay) so assuming it’s an electric guitar depending on how bad the latency is, you may need to use a Y-splitter to connect an amp.
Or, you could get a separate interface for the guitar and use the mixer for monitoring (mixing the guitar and backing track, again with a splitter.)
If you have your headphones connected to the Mackie, then you need to route the playback from Audacity to your headphones without routing it to the main output of the mixer.
From what I can tell from the manual, each input has an AUX section for setting up a stage monitor mix. Send the playback from Audacity to one of the monitor mixes (AUX1 is always pre-fader), keeping the fader down. Send your guitar to the AUX1 mix, and with the fader up to send the guitar to the main output (and thus to Audacity to record). You then need to listen to the “stage monitor mix” or “AUX1 output” to hear the backing track and guitar together.
The headphones are connected to the Mackie, I’ll have to see if Audacity’s routing options allow for that.
Your second paragraph looks to be the best option. It’s a little over my head, so looks like I’ve got my work cut out for me. I’ve never had an instance that would require to use the AUX configuration. I’ll give that a try.
There isn’t much for a Mackie forum out there, other than Gearspace, and that’s OK, at best. The YouTube channel I stumbled on describes panning both channels to separate (L and R) on the board, which Inhave yet to try.
It’s funny you should mention latency. I use headphones on the Mackie, meaning I’m monitoring both the USB and electric guitar channels from the mixer. I always have to nudge the recorded track slightly to align with the backing track on Audacity.
I use an active DI Box for a direct guitar input to the board, as well as with my pedal board.
I’ll experiment with your method, as well as @billw58. Both your methods are a bit above my pay grade, so I’ll take the challenge.
Don’t try to monitor with you headphones connected to the computer. There is a delay between when you strum the guitar and when you hear it in your headphones.