Hi,
1 - there is a setting under app settings -> smart inbox to show "unread only", this is labeled as "smart folder time period", and is the first item, you may scroll the choices list
2 - no popups
3 - normally if you reply to a message, you won't be seeing the drafts / sent folders... now if you go back your sent folder to resend a message, then yet, the app stays where it was.
4 - the app itself doesn't care, plays whatever is selected... to be able to select sound files (from the memory card, where you would copy them yourself from elsewhere) -- install a file manager like ES Explorer, Solid Explorer, or the free app called Rings Extended.