What a great space to be creative. I would do the following....
1. Start digging the soil outside of the pond and use the soil to build it up around the pond. at that point you will have a trench outside the pond.
2. In the trench I would add some good compost or fill it with leaves and some garden soil now and let it sit for the winter. By spring
you will have a good medium to plant
. Once you get some plants
in there the soil will not wash away.
I love Hostas, Ferns, some daylilies will grow in dappled shade. Just start going through the magazines and catalogs and finding plants
that like shade that appeal to you.
4. For behind the bench....Some good shade vines Sweet Autumn Clematis and Honeysuckle I love Honeysuckle for the fragrance.
5. The reason people are telling you to seal the logs is so they do not rot. Put some plastic around them and seal them they will not hurt the plants
. Or seal them before you plant
I would use gravel, mulch leaves, stepping stones leading to the bench.
Where in Pa are you. I am in Pittsburgh.