they can only buy back fully if they precipitate other sales from their dump (and the ensuing fear). You can assume that if there's more selling after a dump, if they don't know something the market doesn't (ie pending mtgox shutdown) , then there will be opportunity to buy back more coins for cheaper.
The fact that a few can control the market and manipulate the price
because they own large % of the total BTC, means it's centralized.
That has become less and less true as time went on in our history. Each dump will typically reduce centralization of coins since it is more unlikely that they buy back all their coins+ unless they buy into a precipitated fall (which still, they can't buy into it all at once because that would kind of defeat the strategy of pushing the price down to get cheaper coins) . This move requires impeccable timing and increasing amounts of capital as the wealth becomes less and less concentrated (which it has). As such, the crazy swings we had back in 2011-12 have become less and less frequent.
However, if anyone has a way to figure out if there's been a whale(s) slowly getting larger and larger.... that would be good to know.