Thank you for your post.
Make sure you are using Notepad (Windows) or TextEdit (Mac) when modifying the Assisted Install.txt file. If you are using another text editor, make sure you save it as Text. In addition, do not copy and paste information from another application, as there may be some additional encoding, and this can result in unreadable characters (boxes).
I used Notepad on Windows when modifying the 2 files. Is there some way I can check for any hidden box characters - looking at the files they look fine to me?
Notepad should be fine.
I did some further research, and discovered that using "/qb" command in the setup.ini file causes the Windows installer to display unknown (rectangular box) characters. I'm sorry I didn't see this previously, because I was more focused on the Assisted Install.txt file. The bottom line is that this is a known issue, and I have attached your post to it. If more information becomes available, I will let you know.