Yep, renting sounds like the thing for you. I don't always advocate renting, but this is in cases where neither the prospective tandem or the prospective stoker know if they want to tandem. It sounds like from your original post the two of you (or at least one of you) thinks tandeming is a good idea.
Your specific concerns can be addressed in a single ride. Don't expect things to be perfect. She will wiggle around. You will wiggle around. She will do things that are counterproductive to a smooth comfortable ride. You will do things that are counterproductive to a smooth comfortable ride. The big question, do the two of you like it enough to work through the inevitable beginner issues? A couple of additional rides and you can be well on your way to a wonderful life Two-gether.
Our first ride on a tandem was on a 1970s "wiggler". My wife couldn't peddle without her knees hitting the handlebars. The seats hurt within the first 10 feet. The timing chain had a habit of popping off. We went a total of 4 miles on that bike in a total of 3 rides. Yet we knew from the first ride that we were going to be riding a tandem together.