Ooh, lots of good questions.
Wrt the first, what are suitable topics or subjects? I recommend that you answer this question by asking yourself to identify YOUR greatest areas of expertise, and then build a class around those techniques or types of cookies. Another consideration is whether to give a technique-only or project-based class. If the latter, it also has to include enough technique-based instruction to allow students to be successful with the projects. If this is truly a beginning class for beginners, then you - of course - need to talk about the icing you use, how you mix it, the various consistency adjustments needed for various tasks, and the basics of outlining, flooding and wet on wet. These are the core foundations. From there, distinguish YOUR classes by doing what YOU do best.
I could expound for paragraphs on the other questions as well, but will wait for others to chime in first.