Pausanias on Achaean armament, ca. 200 BCE

[Aryaman2]

So I agree, they aren't part of the phalanx. But I think that's more likely because they sometimes operated outside of the sarissa phalanx, rather than because they never did.

Quickly too
I think you miss the point that Philip rejoin his dynamin with his Ilirians and Peltasts before he recrosses the Eurotas, so the first time the expression heavy armed is used there is already a phalanx force across the Eurotas.
The argument of consistency in numbers for Seleucid argyraspides and Peltasts is very inconsistent.
I would love to see the epigraphic evidence, could you provide a reference?
I agree with you here, but I would guess is the other way around, they are not part of the phalanx because they are trained as light infantry, but if required they can pick up a sarissa and be part of the battleline of the phalanx. I have in mind here the historical parallel of Spanish Halberdiers in the XVI century that fits neatly performing the same multiple role, however I have to see a source that clearly says that Peltasts carried sarissa.