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In the shade, ferns, hostas, astilbe, bleeding heart, shade annuals like coleus, impatiens, etc. Check out how heavy the root system is - digging there may be difficult. The pine will take up much of the water so be aware of that challenge in keeping plants moist enough. For sun, your choices are only limited by your tastes. Since you want to be able to see the plants behind this planting, keep in mind that height of plants will be a factor. You may want shorter plants here You may even want to consider a ground cover - visit a good nursery and ask lots of questions specifically about your area.

Repotting of trees is really no different than most other container plants in that this is normally done while the plants, or trees, are dormant. This makes it less tramatic for the plant. This is normally done in late winter/early spring.

Yes, you can. This article will help you with the soil mix: https://www.gardeningknowhow.com/special/containers/proper-soil-mix-for-evergreen-container-plants-and-trees.htm You may also want to consider heeling in the trees. This is an article on that: https://www.gardeningknowhow.com/garden-how-to/info/heeling-in-plants.htm

Here's a picture of white pine seeds: http://commons.wikimedia.org/wiki/File:Pinus_strobus_seeds.jpg

In this case I would most certainly call in an arborist to correctly determine if the tree is compromised enough to need to be removed.
Many tree wounds can heal and the tree live without issue.

https://www.gardeningknowhow.com/ornamental/trees/tgen/repairing-tree-bark-damage.htm

https://www.gardeningknowhow.com/ornamental/trees/tgen/peeling-bark-on-trees.htm

This article may help: https://www.gardeningknowhow.com/ornamental/trees/tgen/tips-for-growing-grass-under-a-tree.htm