So do the whales have the ability to dump us into a bear market on their own?
If those whales conspire with each other to intentionally bring the entire market down, I think they can. But the question is, are they doing that? I don't think so.
So the most that a whale could possibly do is to trigger the sell season by dumping all of his/her billions worth of Bitcoin at once in the market. That would definitely affect the price. This will cause a lot of negative speculation and rumors and FUD. As a result, the spaghetti hands would emotionally react to the sudden plunge in the price and would immediately create their own sell orders. This is going to cause more damage to the market than the initial action done by a whale.
They would be able to buy cheap Bitcoin but if we're in a bear market they can't sell for huge profits (and then rinse and repeat) until the next bull run. Or is Bitcoin accumulation the be all end all for them?
Buying low and selling high is a thing in Bitcoin. There are others who purely HODL but there are also countless others who buy and sell and make a profit not just in terms of fiat value but also in terms of Bitcoin. That's regardless if you're a whale or a mere fingerling.
I don't think many whales treat Bitcoin accumulation as the be-all and end-all. How many whales truly believe in the future of Bitcoin as it is? How many of them are just riding the hype just to grow their wealth? How many of them are just avoiding the dying fiat without ever understanding the philosophy of Bitcoin?