Is the abhidhammic view on mind an externalist or internalist one? I'd like to try and categorize the abhidhammic view into either as an internalist or externalist or know if it's possible at all, knowing how the historical Buddha himself even refused to either ultimately deny or agree with the usual problem that would easily decide this issue. 

As far as I understand my main problem is that cetasikas seem to imply an externalist view (citta comes into contact with something, they also rise and disappear with citta together) because according to externalism, for a mind content to arise, it is necessary to be related to the environment in the right way.

Citta itself seems to be an intrinsic property though, as far as every agent is capable of knowing something. This strongly implies an internalist viewpoint (there are intrinsic and unique properties of agents that mental contents supervene upon), as our contents are individuated by the properties of our bodies.
The question may be answered directly. Whenever you come across two theories that are contradictory but complementary such as internalism and externalism Buddhism denies the truth of both. This is a general rule and may be safely applied in all circumstances. This is why the phrase 'Middle Way' is said to have infinite applications. 

It is an avoidance of all positive theories in favour of a neutral one. Hence the Buddha's insistence that we avoid all extreme views. 

To explain or understand a neutral position is a major challenge but just knowing what Buddhism says about these things is not difficult. Wherever there are two extreme views both are rejected. This is necessary for a doctrine of Unity since all distinctions and divisions must be reduced.

Internalism and externalism are partial or selective theories and as Kant and Nagarjuna note all such theories are undecidable since neither extreme answer can withstand analysis.