I would say probably no, but you will more than likely have to disclose it. Highly recommend the 7sage admissions course, it's just $10 and has great advice on how to write these.
It depends on the way the question you're answering is worded. Do not disclose unless you need to. You need to read each question you answer carefully.
Can you disclose it without writing an addendum? I haven't submitted any apps yet so I don't actually know. If you can explain it sufficient in the character and fitness portion then no need for an addendum.
I worked as a Teaching Assistant in the same department. Would you recommend I include that? Most likely, I will not include it and rather send transcripts to make sure I disclose all information.