Is the abhidhammic view on mind an externalist or internalist one?
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.