Re: 1 - If I open a book from the "All by Author" Collection sorted by Author, then Options - Back to List, the sort by Author seems to be retained. But if navigating Home - Collections - All by Author, then sort by Added Order seems to be the default starting position. I don't know whether calibre controls this default. All I can see in the media.xml file for Collections (playlist) is title, sourceid, id, uuid. I don't think items 1, 3 and 4 define sort sequence, I'm not sure what sourceid is but it doesn't sound much like a sort sequence. We'd need chaley again.
Re: 2 - I don't have any DRM'd books or Library books, so I was hoping you'd know more than me on that front.
Your guess is as good as mine.
Not much help, I'm afraid.