He might still have some influence over the blocksize debate and voice his opinions, and yes the 1 million coins he is holding could potentially flood the market, which the exchanges can actually help to buffer by setting some rules?
Other than that, bitcoin is basically a trustless system and depends on noone, controlled by noone.
His 1m is now only 15% total supply so the worst could be 15% variation. But as others have said Satoshi if not dead to life is still dead to Bitcoin because he's been out the scene so long now.
your calculations a bit off, as 10% of 15m, is 1.5m,
so he actually has about 7%, but when you consider there may also be a couple million lost coins, its probably about 10%
now btc has taken 10% nosedives, so unless his 1m were added to the whales that currently manipulate the market
it wouldnt be much difference
sure when trading it would make a huge difference for a while but it would be short lived
price would just settle down 10% lower, and then continue as normal.
one thing i really hope is that those at the top of the bitchoin chain,
are not all in together bring out stories for media and pumping and dumping all together
the temptation would be there for a handful to manipulate bitcoin and earn millions
but if this were true and ever found out it would destroy bitcoin in seconds