It sounds like having someone move in works the same way as marrying them, which is good to know, thanks. In my initial attempt, the founder married Agnes at the Art Museum - then I had them return to the founder's lot, and they rode in Agnes' car, which was then available in her inventory.
Meanwhile, I've done some additional research RE nectar made with life fruit, and the sources I found indicate that if it has at least 6 life fruit, it will extend life by one day. I'll assume that information is correct, which would mean nectar containing up to 50% life fruit is allowed.
Thanks again!