It is, because another way of phrasing that is "coresidence provides no additional explanatory power beyond economic circumstances."

When that's what your data looks like, proper study design either involves testing that hypothesis, or staying the fuck away from making conclusions that take one of those as significant and one as non-significant.

Not really, it's normal.

If you have two independent variables that are highly correlated, and you include both into the model, it's going to be pretty arbitrary which one ends up with statistical significance.

If we're dealing with weak effects and small data, there's very little one can do. That's why epidemiological studies like this kinda suck.