Estimating the residence time of atmospheric CO2 from bomb 14C
Judging by the general quality of the first graph, I don't think that it is the most reliable source. However as I don't feel like reading trough all papers, I will accept the numbers as they are stated. However once you include uncertainties of the observed residence time, I would assume most of the studies are not significantly different from each other. 

The variability in the calculated residence time can be due various reasons; the measurement location, the number of observations used and the length of  the observational record. 

The measurement location is probably the most important source of variation as results may vary a lot depending on where you measure. Multiple observation locations measuring what is called background air masses would be needed to get a good indication of the global average residence time of $CO_2$ in the atmosphere. 

I would say you are spot on with the complicating factors. 
Local influences are important as $CO_2$ emissions have different radiocarbon signatures depending on their origin. $CO_2$ from fossil fuel burning contains no radiocarbon while the nuclear industry emits low amounts of pure $^{14}C$ (not necessarily in the form of $^{14}CO_2$ though). The natural sources and sinks of $CO_2$ (biosphere and ocean) are also not in equilibrium with the atmosphere since the atomic bomb tests in 1950s and 1960s. Immediately after the bomb tests, the large amount of $^{14}C$ produced in the atmosphere meant that it was enhanced in  $^{14}CO_2$ compared to the biosphere and the ocean. Since then, $^{14}CO_2$ has decreased in the atmosphere, as it was taken up by the biosphere ant the oceans. This lead to an such a large increase in  $^{14}C$  in the biosphere and the surface ocean, that they are now enhanced compared to the atmosphere. 

This graph including the oceans should explain what I mean.