Source, please? Harry says this in Blood Rites I believe

AFAIK, you are mistaken.  Blampires were created within recorded history (during the Dark Ages).  Specifically -- AIUI -- Drakul was trapped in a Starborn body last cycle, so about 666 years ago; so his son (who created the BC) is younger than that.

I don't think we know, for sure.

Whampires speak Etruscan, which was an early rival of Rome's.  The languages are distantly related (with a common ancestor) rather than either being "a form of" the other one; Etruscan is thought to be the older, with some influence on Roman (and via Roman, maybe even into English!).

We don't know the Whamps originated there, however (afaik, there is no WOJ specifying this); it's possible the Etruscan-speaking Whamps were simply the winners in old Whampire rivalries, and that Whampires pre-date even the Etruscan culture.  But... call it 3000 years ago, if we take the earliest Whamps to have arisen with the Etruscans.

We have a bit more detail on the Reds.  I think it's safe to pin Rampire origins to Mesoamerica (or maybe Polynesia) where they managed (somehow) to capture some of the great Powers worshiped as gods by the early native Mesoamericans (I suspect (but do not know of an WoJ confirming) that this happened around the fall of the Mayas and the dominating rise of the Aztec).  Drinking that super-powered blood gave them a huge power-up, and they stepped into the role of the gods, and were worshiped as-such going forward.

Whamps and Blamps overlapped from the beginning of the Blamps.  There was probably no Ramp interaction until mortals discovered the "New World" & Cortez' conquest.

But before that...?  In theory, either the Reds or the Whites could be much, much older.

Well, LASCIEL will - she just projects to the body thru the Coin... Ursiel gets killed by Michael early on, decapitated IIRC, and HE comes back. Rasmussen, the host, is the one who goes. The Denarians could well move a few tons of rick and pick up Lasciel's Coin from Ascher's dead body, though they may have ways to summon the coin too (I know Lash said she could tell Harry how, but that might be different, as she was already in his head)

Especially because Rivers himself seems at a loss - so it is not a typical Forest People move

Well, they are human enough to breed with humans.( hence River's son) So that could be enough for a coin merger.

Surviving the ice grinders will need some explaining though. Maybe Jim just wasn't done with the character's plot arc.