Prepare presentation for Ph.D. Interview in Germany
No your don't have to know everything, first of all that's impossible to do in just two days, second of all you could not talk about every aspect of the topic in just 15 minutes.

15 Minutes is really short If you want to talk about your experience/skills don't include this into the talk. I guess they want to hear you talk about the topic on the first place so use this time to do so. This is the second round of an interview so the persons not knowing you will ask if they want to know something. Of course you can create slides to do so but I would keep them independent of your main talk.

"A Potential Methodological Approach" Don't talk yourself into trouble here. If you are not familiar with the topic and they even know that I would not risk too much at this point. Just talk about stuff which you are absolutely sure that it's right.

"Expected Findings" This seems quite bold if one keeps in mind that you are not familiar with the topic. Of course everyone can dream about some nice results but without some knowledge of the field this point seems a bit too much. Just say what you want to do.

Other Idea You could include some quite new results. If you have read some papers about the topic maybe there have been some recent results. Just mention them to show that you are trying to get into the topic. (Again just do so if you are sure).

Other than that just try to give a nice introduction to the topic and explain what is known and where you want to tackle open problems. You are not giving a talk about an/your research proposal.