Oh that bit with Achilles is just the definition of 'urgh'. Penthesilea was an awesome warrior in every sense of the word, she definitely deserves to get remembered for that instead of whatever the hell that bit after she died was. And I wonder about that too! I think there's a passage about Priam having fought the Amazons when he was young, so I guess they must have made peace at some point after? If I had to guess, I'd say the Amazons joined the Trojans because, old rivalries or not, the Achaeans were a clear threat to the entire region, so best to join forces and try to stop their advance.