Thanks for the answers to my questions.
For question 1, I created another .au3 file which executes all my au3 files in sequence. In this way, the users (Product QA) can easily modify that file. If there is need to run different set of .au3 files on different products or platforms, I can put them in different config files and read config files according to the test needs. I also created some au3 files served as libraries, just as what you suggested.
Question 2: Looks like window title is easy to check, but the contents are hard, like some java programs. I only get title, but content is enpty.