What questions should I ask a PI in a PhD interview?
Well, depending on the your topic but 1 min per slide seems the norm. However, for a presentation with a possible PhD, you might get a way with less number of slides for more details/discussion (maybe neglect literature review or discuss it while transitioning between slides) since he is familiar with the topic (I'm assuming so). Keep in mind that when I met my current advisor, I met him in a conference and we talked for few minutes (he asked me very detailed questions (2 questions)). so I'm building my answer based on my case.

Regarding your second question, maybe you can ask regarding what equipment do they have in the lab. How many students do s/he have? Do you get to work with the advisor directly? Or senior/post docs and rarely meet him/her? Will you be fully funded (if applicable)? Will be an RA or a TA or both?