It's cheating, plain and simple. Live chess is designed to mirror OTB as much as possible. Online chess is designed to emulate correspondence chess. Different rules apply. Let me get a link:
https://support.chess.com/customer/portal/articles/1444774-what-are-the-rules-for-playing-
Pretty sure its not - after all, its standard to use em in Online mode. The only real difference is that in Live, the time it takes to use em makes it generally not worthwhile.
I do it more to learn/refresh myself on openings, not so much to help with performance. Since I play 5/2, I'm not sure whether looking at a book or 365Chess helps or hurts.
I'll say this. I often have a rating-destructive urge to mess around with openings I dont understand in speed chess, and using 365chess allows me to do this without the standard rating penalty I take when doing this. So in that sense, it does help.