Why dump? A shadowy figure with deep pockets need merely to offer BTC to big players at MtGox price, rather than dumping them on MtGox.
Because it's much easier to manipulate a $350 million market vs a multi-billion market. And MtGox has the volume needed in this sort of scheme, you can't get rid of that many coins anyplace else as easily.
Either BTC is here to stay or it's not. If it's here to stay, it's easily worth billions of dollars as a phenomenon, thus, "dumping" coins at $30 (that is to say, the bitcoin revolution as we know it is only worth $350 million) would appear sort of foolish.
Irrelevant. This is looking at it from the standpoint of someone like you or I, who wants to see Bitcoin succeed in the long-term. Someone with big pockets could just see an opportunity to make a quick buck. Their bullishness/bearish outlook on Bitcoin doesn't matter if all they're looking to do is make a few hundred thousand and move on.
I'm not saying this is actually occurring, I have no idea and as Bitcoin's market cap (for lack of a better word) increases, it further distances itself from being the low hanging fruit by potential market manipulators. But it shouldn't be something easily dismissed either.