Hi Chris, I've wept and gnashed my teeth today and made some progress, all the ins-and-outs of which I won't bore anyone with. But I can compile the program, create a installation package, install it, and it seems to work.
I only have an Arduino Mega for testing, I don't have a TCB or even an RC radio. So for testing I am only able to confirm that I can connect to the processor (as you say, rather flaky this), load firmware, and read/write settings.
The problem I have yet to overcome and which I've given up on for today, is related to what you described as Blooming Windows Security, though in a different place.
The new version of Assistant (I am taking the assistant.exe from Qt\6.8.1\mingw_64\bin\, I assume this is what you did?) doesn't access the .qhc file directly that we so conveniently provide in the "help_files" folder, but rather
as described on this page creates a copy of it in a "cache directory" which is a subfolder in "help_files," created when the Assistant is first called at runtime. Of course, "help_files" is a subdirectory of our program installation folder, and Windows does not permit the creation or modification of files within the Program Files directory, so this fails.
That link I provided tells us that we can modify the cache directory if we want, in the XML .qhcp file. However, that doesn't help us, it just changes the name of the cached subfolder, which will always be in the same folder as our .qhc file, it doesn't refer to an absolute path somewhere. Even if it did, there is no way I can see to point it to something like the Windows environment variable %LocalAppData% because this is an XML file and not a script, so that variable is not going to be parsed (I tried).
You'd think this would be a problem for the whole world but Google has not shown me other users who've struggled with this same issue. Maybe no one is using Assistant.
It seems strange you would not have experienced this same thing? Assistant works fine when I'm running from the debug/release folder created by QT Creator, because there we don't have file write permissions, but once installed in Program Files it doesn't.
The good news is it will not be a problem to divert our downloaded Hex files into the user appdata folder, though I haven't yet made that change.
I committed some changes to your fork (strangely I seem to have the permissions to do so), these are just fixing some fonts and formatting that seem to have been changed by the new Qt Creator.