Is there any way to delete photos from the main Photos library without deleting them from any or all of the albums and/or folders?

On macOS 10.13 you can hide photos. Then they will not be visible in the Photos view, but still be visible in the albums.

1. Select the photo or photos, choose Image > Hide [number] Photos, then click Hide Photos.
2. The selected photos disappear from your moments but are not deleted. 

To show hidden photos, choose View > Show Hidden Photo Album. The Hidden album appears among your other albums. To unhide photos in it, select them, then choose Image > Unhide [number] Photos.

No. photos in the album or folder pin points back to the photo in the main library.

No. Note that they don't take up any extra space. There is only one photo even if it is displayed in multiple places.