Is a weak deactivator considered a stronger activator than a strong deactivator for the purposes of directing effects A nitro group is a strong deactivator whereas a halogen is a weak deactivator. The directing effects of these groups oppose each other: Nitro is meta directing whereas halogens are ortho/para directing. It is said that the strongest activator determines the directing effects in a disubstituted benzene ring. In a ring with no activators, such as one containing a halogen and a nitro group, is the weakest deactivator considered the "strongest activator", so that in the example I have just given the halogen would control the directing effects and said ring would be ortho/para directed?
I think you are reaching the limitations of directing group rules. These rules are not perfect and can often break. 

In this example, chloride and nitro meta to each other, the group should add meta to the nitro group because the nitro group has a much more significant effect on the electronics of the molecule.