Hi Dominika, and great work. "attitudes" is a classic qualitative research question word. Significance is very well written with the call of an international body.
re: your challenge #1, you could consider dividing focus groups by experience of the participants with PrEP
I like the idea of the knowledge component - you may also get a sense from how people talk of what they understand - or you could even ask them to describe their understanding as part of the interview.
I think you will probably get an honest appraisal of challenges if you base your interviews and focus groups on participants' actual experience.