Eh, I have far better things to do than rewrite an entire section of Wikipedia. I'll just stick to scholarly works on the region and topic.
It's lovely if you have access to works in subjects that are pretty specialist. There is a relevant and interesting work on the subject of Greek and Indian thinking mash-ups:
“The shape of ancient thought. Comparative Studies in Greek and Indian philosophies”, by Thomas McEvilly (2002)
You actually disparaged in this thread a wiki article that references him.
I fully acknowledge that Greek culture played a big role in Gandharan Buddhism. But to call it Indo-Greek is to ignore the people who lived there, other groups that moved into the region, etc. For instance, if the Greeks were so important to some of the changes that Gandhara witnessed (such as the depiction of the Buddha in human form), why does this only happen after the Greek states fall?
I don't think calling a polity Indo-Greek is off the mark when it was founded by Greek adventurers and retained a Hellenistic ruling class while they recognizably existed. Sure, the bulk of the populace was local Indian by ancestry and culture. Just as most of the Asian polities founded by Alexander, the Diodochi, or their descendants were a Greek crust on a filling of local cultures.
There is the very good question of the chronological gap between the end of the Indo-Greek kingdoms in Gandhara and the rise of the so-called Gandhara school. Of course, just because a Greek ruled kingdom dies doesn't mean that its craftsmen, its aesthetic traditions, and even its religion, dies. There were almost certainly Greeks (either by ancestry or culturally) who continued to live in the area.
The best theories think it was the combination of a lingering tradition carried over from the local Hellenistic influences, leavened by Parthian contacts (which were in themselves Hellenistically influenced) and a new infusion of Hellenistic influence from Rome related trade contacts ---all in the service of the predominant local Buddhist culture.
It is fascinating to see the evolving anthropomorphic representation of Buddha in these centuries.