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.
This may have happened when the price of bitcoin was much lower (and the block subsidy was double what it is now) as much less money would have been involved. I would highly doubt someone would attempt to draw the price lower after investing ~$5 million.