There is attested contact between India and lands west (loosely “Persia”) from the time of the Indus valley civilization, over a thousand years before the Buddha. The subsequent Indo-European culture is, of course, shared between the two lands. There are a number of references that support the idea of contacts at the time of the Buddha (Analayo has a nice essay on this), but personally I think no special pleading is required: why wouldn’t the Indians have had some idea of such foreign lands?
Pali yona is from Ionia, and is a general term for the Greek cultural sphere. Kamboja is less certain, but I think it’s likely to be an equally general and vague term for the lands between India and “Greece”, for which “Persia” serves no worse than any other. Compare the Greeco-Roman use of say “Ethiopia” for all lands in Africa or “India” for south Asia.
Seems unlikely; Cambodia was named after Kamboja.
Again, seems unlikely at this point: not long after, of course, the Greeks were in Afghanistan so this would be correct then.