The range of people that sensē (sensei) covers is quite large; not only teachers and professors but also laywers, doctors, politicians and lecturers. If you are going to see the nun at her lecture, this address form is very fitting.
Religious figures may have different forms of address depending of their sects and positions in their denominations. One of the most general address form is 方丈さん／方丈様 (hōjō-sama/san). This is used for senior Buddhist monks regardless of their sex.
I've never met her before. But I'm going to interview her about the books she has written. I don't know which language we are going to use. English I suppose, but there could be an interpreter. Anyway I couldn't use words like 'madam' or 'mrs'...
Thanks for your precious help.