Yeah, no arguments on it being available more widely than I thought, your prompt motivated me to look into and I independently confirmed what you said, I just didnt know that much change was allowed. Perhaps it was my old habits that meant I never noticed how much you can do.
I think its been explained though that the XAML file isnt ran directly, its translated into .dlls that can ‘look’ quite different in raw code and so its quite hard to do what you ask I think.
I’d say look into test cases, its not so much about me saying it will fix your problem cause your problem is test data, but its about being able to run and re-run small workflows easily, set up complex data types as you indicated was a challenge for you and do a quick dev > test > dev cycle. The scenario of messages being bad etc can be quickly tested fixed and iterated in this approach.