5. They have built a centrifuge which achieves the expected g loadings.
Where did you get that? In the videos I've seen they only claim to have made something operating at a fraction of the stresses they need to achieve. They makes some solid claims about the potential strength of carbon fibre arms. They do some hand waving about what seems the biggest issue to me - the period of enormous imbalance they seem to have no way to mitigate.
I believe in the video they claim to have reached those G forces on a smaller scale Spinlauch or As a recall they were doggie about it saying they can't show it/talk about it as they way they are doing it is a trade secret. That's the same bullshit line Elizabeth Holmes and Trevor Milton used. And as we now know, Elizabeth Holmes and Trevor Milton were full of shit. Spinlaunches claim sounds extraordinary. And as Carl Sagan said, "Extraordinary claims need extraordinary evidence." So far there's been no evidence to support their claim.
They had a view of a centrifuge room. It appears to be a centrifuge roughly two meters in diameter. They asserted that they were running this at comparable loads to those used in the real application.
For cross checking numbers the acceleration is velocity squared divided by radius. In the video they claim a Mach 5 launch velocity as I recall, which is roughly 1600 meters/sec. My recollection is their target radius was about 40 meters. This gives 64000 meters/sec*sec or about 6500 g. So 10,000 g would be an appropriate design goal to give some safety margin. Those with sufficient interest can go back and watch the video carefully and get exact numbers.
For the smaller scale centrifuge this requires rotating at a bit over 30,000 rpm for the predicted load, and 38,000 rpm for the design load. Not trivial but doable. Needs the carbon fiber and needs the vacuum, but on a far smaller scale.
Extraordinary claims do require extraordinary evidence. But this particular claim is not particularly extraordinary and at least some evidence was provided, in the form of photos of test articles and verbal descriptions of test results. For another example of the general feasibility of g hardening devices, think of the widely publicized application of bunker busting bombs in the Iraq wars. The fuzing circuits in these had to withstand the impact of the bomb with several layers of reinforced concrete a meter or more thick. Counting the impacts as they occurred. And going off when the last layer as identified by the German designers of these bunkers.
You are free to dismiss any and all of that until SpinLaunch publishes full results in a peer reviewed document, which is the standard some set for evidence. People with that standard would not accept a successful launch demonstration because it might have all been some kind of trickery.
I am willing to accept this part of the story because they are doing things others have done, and those doing the discussion of this issue demonstrated knowledge of the real issues in such efforts, and credible solutions to those issues. It is certainly possible that they have missed something, and that updates to their designs will be required. But nothing about the g hardening screams BS. So this is just the normal engineering process, and the whole issue is whether they can do it (design, facilities and operational costs) cheaply enough to make the project economically viable.
Another example illustrates the difficulty of answering the economic question. In the recent past two teams were issued contracts to develop a manned space capsule for use in ferrying astronauts to the ISS. One team was new to the field, with limited experience in the area. The other team had decades of experience. One of those teams completed the task, more or less on schedule and budget. The other is still not done, has blown past many schedules and reschedules and enormously outspent the other team. The real world delivers many answers. All seem somewhat obvious after the fact, and there will always be those who are there to tell you they knew it all along.
I remain skeptical on the financial side of this, but would not be flabbergasted if I am proved wrong. I don't have any emotional investment in either the technical issues or the financial issues.