OP wrote:
"2. Once all my photos are in the app, can they be restored back to files in finder?"
Are you really intending this question to mean something like:
If I currently have all my photos in an ordered "folders and files" arrangement, can I get this arrangement BACK after migrating everything to Photos and then deleting my pics from the finder?
I would think that the correct answer is, "no".
Because although you can export photos "out of" Photos, I don't see any way you could export groups (or large groups) of photos out, and "into" the original folder/file hierarchy from which they came.
In other words, Photos (like iPhoto before it) creates its own "library" of originals, but it does so using its own database arrangement that is nothing like finder's.
I think your better option would be to use a "referenced library".
Under this scheme, Photos will leave your originals "where they are" on your drive (presumably in an orderly folder/file arrangement), and create its own "library" of previews and information (so that it can display the edits you make within it).
There is an option in Photos' preferences to set this.
Personal experience:
I'm hopelessly "old school" in that I prefer to use a structured "folder/file" arrangement for all my photos, music, etc.
I want to be the one determining the order in which folders are structured, and to which folders go the files. I'm not interested in a "database" arrangement as such is created by Photos, iPhoto, iTunes, or whatever, that makes "going back" to a finder-style structure of file placement all-but impossible. But that's just me.
Your needs may be different, and probably are...