It's difficult to know how different the 500TPA and 2000TPA...

It's difficult to know how different the 500TPA and 2000TPA plants were in design, because
A) We were told the pilot plant was a success so they can't admit to making significant changes without directly admitting their lies, and
B) The functional problem was the same with both plants, so any changes made if any (presumably there were some) in the fundamental design had no impact, meaning they were either trivial or functionally identical and something entirely different needs to be done.
We just have no way of knowing and the company has put themselves into a position where it's difficult to admit to anything or give much detail without losing face.

It's difficult to be 100% sure, but unless you believe the entire design is flawed, in which case the previous plants are useless anyway and a new pilot plant is clearly required for a fundamentally new design, it would presumably be much cheaper to modify the relevant area of an existing plant, especially the smaller 500TPA plant, than it would be to build an entire new 10,000TPA plant which they say is pretty much going to be the same design but scaled up anyway!

However you slice it, this screams 'stay away'.z

And absolutely positively, they can not build a new pilot plant without destroying any illusion of remaining credibility, so they won't. I'm not for a moment saying they will do it, I'm saying what they should do. I don't think there is any significant likelihood of either of the previous plants ever being restarted in any scenario. I'm sure the company will want to go for a path involving ongoing CRs to prevent themselves from drowning and if it ever becomes possible, which is unlikely, they will obtain funding to build their 10,000TPA plant, which will have similar chances of success to the previous two demonstrations of their capability. Having said that, if they do obtain funding, I'm sure after the initial price collapse to a fraction of what it is now due to extreme dilution etc, there will be a period of optimism and price increase (up to 0.5c or something) then a long orphan period then another spike due to optimism around the time of commissioning, followed by the same pattern we saw with the commercial production plant AKA demonstration facility. In the highly unlikely event of funding somehow coming for a third plant, I'd gladly bet on it being their last attempt before any last shred of hope was gone. It's quite fascinating watching the remaining hope in the remaining shareholders at this stage. I can understand hoping for a miracle while on the sinking wreckage of a clearly destroyed ship with no chance of rescue, but it's puzzling to see people choosing not to take the lifeboat and voluntarily going down with the ship filled with optimism, laughing at and mocking the people paddling off to safety, pointing out that there's still another lifeboat they can jump into if they're willing to abandon their luggage on the already submerged deck... and if the destroyed ship does float back to the surface by some miracle, those on the lifeboat can still merrily paddle back and reboard.

It's quite surreal to watch.