It wouldn't be very smart to buy 10,000 BTC and then try to cause the price to fall in order to buy more at a lower price.
Depends really. It could be. People have done things like that often. Especially when there was less liquidity and the market was smaller. I don't know how effective such manipulation tactics are these days though.
Doubtful. Theoretically, it doesn't work. Even if you ignore the theory, it is still very risky and unlikely to be profitable.