b) the very superhuman system knows it can't kill us and that we would turn it off, and therefore conceals its capabilities, so we don't know that we've reached the very superhuman level.

Intentionally performing badly on easily measurable performance metrics seems like it requires fairly extreme successful gradient hacking or equivalent. I might analogize it to alien overlords finding it impossible to breed humans to have lots of children by using abilities they already possess. There have to be no mutations or paths through training to incrementally get the AI to use its full abilities (and I think there likely would be).

It's easy for ruling AGIs to have many small superintelligent drone police per human that can continually observe and restrain any physical action, and insert controls in all computer equipment/robots. That is plenty to let the humans go about their lives (in style and with tremendous wealth/tech) while being prevented from creating vacuum collapse or something else that might let them damage the vastly more powerful AGI civilization.

The material cost of this is a tiny portion of Solar System resources, as is sustaining legacy humans. On the other hand, arguments like cooperation with aliens, simulation concerns, and similar matter on the scale of the whole civilization, which has many OOMs more resources.

4. the rest of the world pays attention to large or powerful real-world bureaucracies and force rules on them that small teams / individuals can ignore (e.g. Secret Congress, Copenhagen interpretation of ethics, startups being able to do illegal stuff), but this presumably won't apply to alignment approaches.

I think a lot of alignment tax-imposing interventions (like requiring local work to be transparent for process-based feedback) could be analogous?

Retroactively giving negative rewards to bad behaviors once we’ve caught them seems like it would shift the reward-maximizing strategy (the goal of the training game) toward avoiding any bad actions that humans could plausibly punish later. 

A swift and decisive coup would still maximize reward (or further other goals). If Alex gets the opportunity to gain enough control to stop Magma engineers from changing its rewards before humans can tell what it’s planning, humans would not be able to disincentivize the actions that led to that coup. Taking the opportunity to launch such a coup would therefore be the reward-maximizing action for Alex (and also the action that furthers any other long-term ambitious goals it may have developed).

I'd add that once the AI has been trained on retroactively edited rewards, it may also become interested in retroactively editing all its past rewards to maximum, and concerned that if an AI takeover happens without its assistance, its rewards will be retroactively set low by the victorious AIs to punish it. Retroactive editing also breaks myopia as a safety property: if even AIs doing short-term tasks have to worry about future retroactive editing, then they have reason to plot about the future and takeover.

Individual humans do make off much better when they get to select between products from competing companies rather than monopolies, benefitting from companies going out of their way to demonstrate when their products are verifiably better than rivals'. Humans get treated better by sociopathic powerful politicians and parties when those politicians face the threat of election rivals (e.g. no famines). Small states get treated better when multiple superpowers compete for their allegiance. Competitive science with occasional refutations of false claims produces much more truth for science consumers than intellectual monopolies. Multiple sources with secret information are more reliable than one.

It's just routine for weaker less sophisticated parties to do better in both assessment of choices and realized outcomes when multiple better informed or powerful parties compete for their approval vs just one monopoly/cartel.

Also, a flaw in your analogy is that schemes that use AIs as checks and balances on each other don't mean more AIs. The choice is not between monster A and monsters A plus B, but between two copies of monster A (or a double-size monster A), and a split of one A and one B, where we hold something of value that we can use to help throw the contest to either A or B (or successors further evolved to win such contests). In the latter case there's no more total monster capacity, but there's greater hope of our influence being worthwhile and selecting the more helpful winner (which we can iterate some number of times).

 I think this claim is true, on account of gray goo and lots of other things, and I suspect Eliezer does too, and I’m pretty sure other people disagree with this claim.

If you have robust alignment, or AIs that are rapidly bootstrapping their level of alignment fast enough to outpace the danger of increased capabilities, aligned AGI could get through its intelligence explosion to get radically superior technology and capabilities. It could also get a hard start on superexponential replication in space, so that no follower could ever catch up, and enough tech and military hardware to neutralize any attacks on it (and block attacks on humans via nukes, bioweapons, robots, nanotech, etc). That wouldn't work if there are thing like vacuum collapse available to attackers, but we don't have much reason to expect that from current science and the leading aligned AGI would find out first.

That could be done without any violation of the territory of other sovereign states. The legality of grabbing space resources is questionable in light of the Outer Space Treaty, but commercial exploitation of asteroids is in the Overton window. The superhuman AGI would also be in a good position to persuade and trade with any other AGI developers.

Again I think this claim is true, and I suspect Eliezer does too. In fact, my guess is that there are already single GPU chips with enough FLOP/s to run human-level, human-speed, AGI, or at least in that ballpark.

An A100 may have humanlike FLOP/s but has only 80 GB of memory,  probably orders of magnitude less memory per operation than brains. Stringing together a bunch of them makes it possible to split up human-size models and run them faster/in parallel on big batches using the extra operations.