Be careful! The Indoeuropean language group will have cognates all over the place: whether the Parthians in any way relates to Partha except linguistically is very doubtful. About the leader Indra destroying Dravidic forts, I believe there is very little hard evidence that indra was an actual person as opposed to a personification of natural forces or that the forts he destroyed, even if they were real forts, were made by people who spoke the Dravidian language. I do tend to agree with you that it is indeed a very good guess from what we know today that the pastoral group who composed the Rgveda came into conflict with an urbanized people speaking the Dravidian language, but whether the allusions in the Rigveda are anyway related to this clash of cultures is highly doubtful in my mind. Personfications are indeed expanded on the memory of real events, but the literal identification of the vedic indra with a person (breaking down mountains to rescue cows) is a bit far fetched: it is easier to do so for the later Puranic mythological indra, but that is easier seen as an elaboration of the vedic indra in an age of empires. In any case, as I have discussed before, the name Indra has a clear meaning which fits in with the earliest mythological use, and the later development to the head of the pantheon is not extremely surprising either: all the above is interesting but not relevant, in my view, to finding the origin of the name. So, to get back to names, a theory such as Partha being related to the Parthians needs more evidence than you presented. The same is true of Maninder, though here I tend to agree with you: at the very least, one can check for early uses of the name (we do know some names from stone inscriptions back to three centuries before the common era) and see if it appears at the time and place which makes a Bactrian influence plausible. Names which have been in use for a while often get distorted: thus Alexander becomes sikander by a simple sound transformation, so matches need not be exact: but evidence in some form is required.