Two things to note here.

1)
The scales on the Inputs and Masters are different. Inputs go up to 0bdfs and are marked as such. Masters go to +18db (0dbfs) with a Unity mark (0db) at the -18dbfs point.

2)
You can't do simple gain math on a compressor using peak values as the attack time renders any gain reduction value void. eg: a drum with a fast attack will go right through a compressor with a "slow" attack setting, say 10ms, with no gain reduction at all...

that helps.
there must be a benefit to different scales in the same unit, but so far it escapes me.
so I will try no compressor and they should be about 18db different?
no compressor, difference seems close to the 18 db difference.

voice,folk
-54 db threshhold
7:1 ratio
0.2 ms attack
2.5 ms rel

ch 1 shows ~50 db compression in the compressor
and the output is just above the 0db mark.

-12 db signal with 50 db compression above -54db threshhold
implies the output should be much lower.

if I put a 16.4.2 in the line with those compressor settings,
what do you think the output might show?

I think if I had -100db threshhold and 100:1, output would still be high

everything seems to overdrive the output.

the question no one has yet to answer
is what is the output of the channel strip,
with -12 input, and the compressor above?

as far as I can tell, no one can tell,
because there is no post fader meter.

The reason for the different scales is quite simple and logical.

The input scale reads dbfs, (DB Full Scale) where the top (0dbfs) means the a/d converter has maxed out and run out of bits. This is the most important meter in any digital mixer as its the limiting factor on how hot an input signal can be. Input converters usually use 24 bits.
Once in the digital realm, processing is generally done using 32 bit audio, for maths efficiency. This gives greatly enlarged headroom and more gain can be added to what would otherwise be signals that were already near clipping. Most DAWS these days can work internally with 64 bit audio for other reasons, which gives other advantages. But compared to the original 24bit audio, the internal headroom is effectively unlimited. ie, you're not going to clip by turning up the makeup gain on the compressor or by boosting an eq band by +12db. Something that could happen in an analogue desk....

As far as the outputs are concerned, the scale is based on the analogue output level. The trimmer can boost the max output to +24dbu but its intended as a nominal +18 max, as expected for most analogue gear (amps, drive racks, etc) that it would be connected to. (though these days most gear seems specd for +24). The 0dbu mark on the output meter represents the nominal analogue level for "normal" working.

The scales on the input and output meters are unrelated to each other in any way. They are measuring different things for different reasons and should be read separately and in context.

The "correct" way to use the input meter is to adjust the preamp so that the level bounces around the middle of the scale. Don't get hung up on precise values, music is dynamic. Unlike analogue you gain nothing by running the inputs hot, but you still raise the noise floor (later) by running them too quiet.
With your channel gains set up like this you should end up with a reasonable signal level hitting the master bus, at which point you can use the Master fader to set the final level. The master meters should be bouncing around the 0db mark. If the result is way to loud, turn your amps down...