Partial answer because
based on a simulation
doesn't address the secondary question about changes with throttling
The paper CFD SIMULATION OF A LIQUID ROCKET PROPELLANT (LH2 /LOx)
COMBUSTION CHAMBER shows a "flame front" in the sense that majority of the combustion reactions take place in a relatively small area of the combustion chamber.
This doesn't have a simple answer.
From a pure chemistry standpoint you can observe that one CH4 molecule will completely react with two O2 molecules. CH4 has a molecular weight of 16, O2 has a molecular weight of 32. Thus you need 4x as much O2 by weight.
However, there are two other factors involved:
You can actually end up with more thrust with a bit ...