Some answers to your first question you can find here: https://madaidans-insecurities.github.io/guides/linux-hardening.html
For the second question about in what ways Secureblue do mitigate that you can find more here: https://secureblue.dev/features
The last question about usability, is very usable. If you use Bazzite you may have a similar experience. It is not like QubesOS that isolate all processes making it even not able to use a GPU.
I’m following your path jumping on Secureblue, because I found the project philosophy appealing to my interest.
I don’t feel the same about the others Atomic distro. I’m probably missing something but the other Atomic projects don’t seem to be adding much value for workstation home users if you know your thing.
Also, to the OP, reading the comments it seems clear to me that even with the best product you won’t be able to learn a lot about everyone’s motivations to not follow the Atomic trend. Although it definitely plants the interest on some that are coming across the topic for the first time, which I think is good. Learning something new should be of everyone’s interest.