I really appreciate that you've put that much time into studying this issue for me.
However, I simply downloaded the actual ISO just this afternoon, before going out for some chores.
So for now, I would like to close out this issue as DEFERRED/UNRESOLVED.
I want to focus on the fact that my reports of no internet access during the install process. Those were being ignored, without justification in my view, because I was not on the latest and greatest daily build. So with this new ISO, I can verify and report if the issue persists.
Thanks again, Chris. Really much appreciated.
As for your question ...
I was using my installed primary environment, UM 22.04.5 LTS (not any Live UM), to attempt the zsync download approach. Until I actually switch to UM 26.04 as my full-time environment, I will hold off any further attempts at zsync downloads.