The back button behavior is actually my main annoyance at the moment.
The configurable swipe is a just a nice bonus that could be possible by freeing up the current swipe behavior.
I don't need to swap albums nearly as much as I want to go back to the song list to select a different song in the same album.
And although it's not draw on the flowchart, you can also easily exit the app from anywhere by just using the home button so you don't need to go all the way to the library just to exit. So I use the "Start at library" option.
Having home button as a quick exit the app option and back button as a undo/return to previous menu is how I use the button in most if not all apps so this is what I feel is the most natural to me.
I need to make a conscious effort to remember to swipe down from "now playing" and then use back everywhere else.
Too often have I mistakenly pressed the back button to be sent back to the library and have to navigate myself back to the song list.
It also seems redundant having two options going back to the library in this mode.