John,
The Mackie mixers have an alt send that can be used as a separate mix. What model are you using? I can help you better if I know specifics.
The guitar sound can be improved with proper gain structure.... balance out the volumes, and if that still isn't enough .... maybe a limiter is needed to squash the signal from overloading the mic input. I don't play lead too often, so I just strum the rhythm at a reasonable volume and it seems OK.

I'm going to star a new topic with some particular questions I have as well ....
Good luck !