I think you have a lot of evidence pointing to the turbo as an issue, specifically the wastegate area. Perhaps a problem from the get go on this new turbo.
I think you have 3 options:
1) wire the wastegate shut to make sure it's "closed" and try your test again. I suspect this will lead to the same result and verify a defect in the wastegate area.
2) remove the downpipe bolts and drop the downpipe down. inspect the turbo, especially the wastegate area for how well it looks to be sealing and how the wastegate actuates.
3) take it back where it was installed and explain that it never felt right and that you've been troubleshooting the boost control, but now it looks like even with nothing pushing the wastegate open, when you should be easily accidentally overboosting to 20+ psi, it's not holding even 10, and see if they'll do some troubleshooting for free since they installed it and it's clearly not working right.
At some point you also might think about visiting the dealership you took it to. The service rep should know that they had a bad diagnosis and you know it. They may give you some money back, they may not, but at least you should let them know that they were wrong and you know that they were wrong.