Mr positivity appears, with his own negative spin on things.
I'm not trying to incite a fight, I'm extremely pointing out cases where I disagree.
Welcome to church audio, where you have six guitars, four(teen) vocals and an orchestra. Show me a console with 70 faders on the surface that lets 3 separate people mix and I'll show you people who might get close to having every instrument audible.
But a 5 piece guitar/bass/vocal/key/drums? Turn it all up, no problems.
Vocals should be above everything, at least marginally. And not every style of music calls for drums as loud as bass or guitar as quiet as keys. Funniest thing I ever heard about sound is "nobody goes home whistling the lights" because they all go home humming the vocals. Or maybe playing air guitar.
3) Partially disagree
This is more a question of the music. If a guitarist plays a phrase between vocal parts, and plays notes behind the vocal, you need to ride them to make sure it fits. But don't push it too hard: it shouldn't take over unless it's meant to be a solo.
You can automate this, by running the guitar part through a compressor with the vocal part acting as a sidechain input. Now when the vocal kicks in, the guitar will get pushed back by default.
Maybe works if you have a top class PA. I find it better to aim for "make what's on stage sound loud enough" because not all bands (can) play CD-quality when live, especially not over the standard Peaveys-on-sticks type of PA at 90-100dB.
Meet PA tuning. Installed systems can be made to fight this, touring acts don't have the time to deal with that in one night. Line arrays can help in larger rooms by compensating for the volume loss front-to-back.
No monitors means no timing. Lousy monitors means timing will be OK but they won't have fun with the music and they won't be as confident.
7) Cut away anything that covers up your vocals, or more important instruments. Be careful if you're also on monitors, because that'll screw those up.
Mostly between rehearsal ending and show starting. Sometimes this is intentional, in which case, put the compressor away. Less skilled vocalists will do this a lot though, so squash EM.
Without any makeup gain, the background detail will stay at the same level regardless of how hard the compressor works. Only when you add makeup gain (normally equivalent to pushing the fader) do the details become louder. Of course this is sometimes necessary if you compress 10dB off the vocal.
*Technicality - if the compressor is still applying gain reduction after the input signal drops (i.e. release time), the background detail will actually be turned down until the compressor is fully "released". But over that time it'll ramp up in volume and this causes a perception that it's increased in volume.