Question: Can someone change Bitcoin's market price or its protocol? If so, who could do that?
Bitcoin market & community is powered by its users only.
Thus, no one can directly affect or change the market price.
And no one can force a change of its protocol, Bitcoin can only work correctly with a complete consensus among all users.
Although the market of Bitcoin isn’t so deep like national currencies (EUR, USD etc…), thus a large amount of sell or buy could comparatively easily affect the market price.
Also, while trading with a Market Maker broker or exchange, it is possible that the offered prices are modified without prior notice.