I'd buy at $300, but I'm very conservative in terms of risk so prices will likely rally long before my mark is hit. I suspect you're correct in that BTC will fall below $400 USD because the daily exchange rate has a lot of momentum that isn't visible within hourly trades due to arbitrage or the lack thereof.

I don't think there's a single entity manipulating the price, i.e. a large oldcoin dump. More or less the market = lemmings off a cliff. If an exchange were insolvent (and this were public knowledge) then we'd see a massive price drop in that exchange, and lesser drops in solvent exchanges, as happened with Mt. Gox.