This may help.
https://helixhelp.com/
I know what you mean, the Princess (princeton) doesn't have a mid, but line 6 does. My logic says leave on the default setting (ie, princess is at 7), then only use bass and treble? Im not sure, it's a good question. Interesting you mentioned the Champ, so many different versions. I think Line 6 champ is based on the 57 or so tweed? only a volume control, but we still have lots of "tone shaping" with the full controls.
What it do know is Line 6 does not include the amps effects (reverb, vibrato, etc) you need to add them as effects after.
Edit: Can't help with your second question.