Thank you all for your input. We are just back from our trip and I am posting this “report” in case someone else has the same question in the future.
The B&B owner has a map that shows all the various ways to get from the hotel to the island (wish I could post a picture of it). To access the walkway along the Couesnon dyke/river you have to walk (3 minutes) along the road toward “Alligator Bay Reptile park” where you can cross the street on a cross walk and access the river walk at the back corner of the L’antre Terre restaurant parking lot. That 3 minute walk was fine - there is a dirt path worn in the grass and we felt perfectly safe as cars passed by us. The walk along the river (paved path with benches) provided amazing views; very glad we did it. From the hotel to the entrance steps that lead up into the Abbey itself took us 50 minutes.
Coming onto the island we took the “back door” archway entrance on the left recommended by Rick/did not go up the stairs and down the main street to avoid the thick crowds. Steep ramps and some stairs but not as many people and some nice views. We did walk down the main street and check out shops as we left.
When we came off the island we took the shuttle to the parking lot and then walked along the road back to the hotel (note if you get off the shuttle at the bridge you can easily access the river walk and get back that way). We walked on the same side of the road as our hotel, facing traffic. Sections of this walk had a worn dirt path but sections did not (you could walk through long grass or in the street). It was ok but not as safe feeling as the short walk to Alligator Bay (though it did allow us to stop in L’Atelier Biscuit shop and pick up yummy cookies).
At night we opted to drive to the restaurant (they offer free parking to patrons after 6pm - get access code online before going). After a delicious dinner we left our car in their lot and walked (sidewalks at this point) to the bridge to view MSM at night. Note: it was light out much later than we expected; dusk started about 9:30pm (May 10th).
We both decided we loved the daytime walk but were happier driving at night. If you were to walk the stretch of road to/from the restaurant at night be sure to bring a small flashlight to make yourself more visible to cars (there weren’t that many cars at night but it felt like they were driving faster than those during the day/not expecting to see people on the road).
Highly recommend both the B&B (rooms 1, 2, 3 and 4 have private patios. We were in 4 and enjoyed relaxing outside with a glass of wine) and the restaurant (the duck was very tasty and good desserts).