Gawd, I pity you, with all this good info at once. But a little more basic theory might help:
OK, I view an OP AMP as a super gain device, (way more than the circuit it will be in). Think something like 100,000 X gain. So, the trick is to utilize feedback, from the output and consider this output as 'controlling' the OP AMP minus or negative input, which it does, but via some reduction.
So, if the positive input (feeble) rises a bit, the huge gain cause a big rise in the output, which also gives a feeble rise in your negative input (you connected it to drive a reduced output to the negative input).
That's a control type response, where the OP AMP fights to eliminate the (voltage) difference, between the two inputs. That is the circuit, with the negative feedback is arranged to do this.
So you never see that huge gain, directly, just think of it as strongly keeping the two inputs equal.
Now, for DC, looking at previous examples, your desired negative feedback is for one to one 'gain', or just have your DC level at about half way, like 2.5 volts if you are using single 5 V supply. The OP AMP will fight, strongly, to control your negative input to also be at 2.5 V steady. Except for noise, there.
Your negative input, from a DC standpoint, should just follow the output, no reduction, and that is what the other posts do show. Therefore, you will see 2.5 V DC right there on your output, too.
You can see, the capacitor keeps the ground out of any DC feedback calculation, the negative input gets the DC 2.5 V since the DC part of the output is not reduced, in that negative feedback.
One way to look at the AC (audio) is that it is a little disturbance, on top of the DC 2.5 V you've set up.
You can also appreciate, a pair of 100 k or a pair of 56 k makes no difference, as long as it's fairly large, so as not to start loading down the feeble AC. You can kind of think if it as two circuits, right there in same place!
Now then, you have a DC level being controlled, putting those 2 resistors dividing the output AC, (because for AC your ground IS connected, so that divides) you have, paradoxically, actually TWO different gain situations, simultaneously.
And one last concept: by feedback having less than the output, you can say it the other way: In other words the output is MORE than the (negative) input, and, hey, same thing can be said about the positive input, your feeble guitar signal. That's because the circuit is controlling itself, to have the positive input be, also, a certain factor or fraction of the output.
Very wordy, and round and round, but it's the key to all those variations being tossed at you.
Question ? I will check back, hope my (limited) expertise helps...