Reading through papers that describe the CO2 measurements, the equipment is solid enough. That's long-settled engineering, and can hardly be disputed without tossing basic physics (e.g. spectral analysis). Then they have to smooth out the natural and man-made fluctuations, depending on the location (some are natural settings like an island in the Mediterranean , some are industrial areas in Germany). In any case they describe their process and in most cases produce a curve that shows mostly the same rise and annual wiggle as Mauna Loa. Remote southern locations like Antarctica produce less annual wiggle but the same overall rise.
It's possible to argue there is a closed circle of science that produces the same results. But it is not possible to argue that the Mauna Loa volcano matters nor that there is a conspiracy of some sort. It could be bad science, but that is extremely unlikely at this point.