You've convinced me that there's actually a lot we can do to test how well current cryonics processes work that doesn't require round trips on human brains.

If we do these tests and adjust the cryonics procedures in response to what we learn, my estimate for the chances that we're not preserving what we need to will probably go down a lot.

They don't all have to go right in the most dramatic 100% manner

Most of the steps I'm on the negative side for are pretty much binary: either they succeed and you proceed to the next step or they fail and you're done. Looking at all the ones that I think there's over a 10% chance of failure, and labeling those where you might get a partial reconstruction:

• You die suddenly or in a circumstance where you would not be able to be frozen in time: 0.1 (partial)
• Some law is passed that prohibits cryonics before you die 0.1 (absolute)
• The cryonics people make a mistake in freezing you 0.1 (partial)
• The current cryonics process is insufficient to preserve everything 0.8 (partial)
• All people die 0.1 (absolute)
• Society falls apart 0.4 (absolute)
• Some time after you die cryonics is outlawed 0.2 (absolute)
• All cryonics companies go out of business 0.4 (absolute)
• The cryonics company you chose goes out of business 0.1 (absolute)
• The technology is never developed to extract the information 0.6 (absolute)
• No one is interested in your brain's information 0.4 (absolute)
• It is too expensive to extract your brain's information 0.4 (absolute)
• The technology is never developed to run people in simulation 0.4 (absolute)
• Running people in simulation is outlawed 0.2 (absolute)
• No one is interested running you in simulation 0.3 (absolute)
• It is too expensive to run you in simulation 0.1 (absolute)
• Other 0.2 (both)

given how long we have avoided either of these fates so far

Humanity is far more powerful and capable than it's been for most of it's history. It will probably get more that way.

cryonicists have added incentive to look at extinction and civilization-wrecking events using near-mode reasoning. This could be a positive externality worth taking into consideration.

I'd be surprised if promoting cryonics beats straight up x-risk awareness advocacy.

The storage space needed (ongoing) should be no more than that of the brain itself

You may be right. If we can get a compact representation this might not be that big at all. At 20 billion neurons with, maybe 100 connections per neuron, four bytes per connection we have ~7TB of information. This is way less than the processing requirements. The WBE Roadmap thinks we might need anywhere from 50 TB to 10^9 TB depending on the level we need to emulate at (p79).