These will all eventually be stolen and recirculated into the supply, possibly causing a huge divergence from users' perceptions about Bitcoin's "deflationary" monetary supply.
At any point, Satoshi's coins could move without warning and be recirculated in to the supply. As could any coin which is considered "lost". We have no way of knowing how many coins are truly "lost", and everyone (at least everyone who understands bitcoin even a little) should know and appreciate this.
We don't. We won't know until after the fact, once ECDSA is broken and nothing is done. It'll be a roller coaster, that's all I know.
Having a sudden influx of a million coins, and therefore a ~5% inflation, would be less damaging to bitcoin than removing one of its core principles by allowing a small group of people to decide which coins are and are not allowed to be used in the future.
The amount of vulnerable bitcoins probably goes far beyond the "Satoshi coins" -- that's just the lowest base line to start our assumptions from.
You can deploy a modified client with a rule that declares some coins invalid, as a part of a new protocol, which would result in a hard fork, correct.
That could be done with a soft fork. Miners have incentive to enforce it since it would prevent unpredictable inflation.