How DSub handles bookmarks:
1) Bookmarks are automatically created for Podcasts, Audio Books, and when the file is longer than 10 minutes.
2) Bookmarks are automatically created when paused or changing current playing. I'm weary about setting an auto bookmark up because this is a mobile device and I don't want to kill the battery/a lot of times users have crap network connection on commutes.
3) There is a button users can press while listening to songs to create a bookmark for the current position. This can be useful if you want to pick up where you left off in an album.
4) Bookmarks are automatically cleared when 84% (to a maximum of 10 minutes) of the way through the file, but only when they remove the file from the list or go to the next file.
5) Created a tab where you can view all bookmarks
6) When resuming from a specific bookmark, it automatically loads the album it is in and resumes playing from the bookmarked position. It doesn't do this if the file is a podcast since those are often listened to one at a time.
7) When pressing play on an album with a bookmark, the user is asked if they want to resume from the bookmark or start over. If they start over the bookmark is automatically removed.

The bookmark comment is set to "Auto created by DSub". I can change that to "Last played by DSub" or whatever to keep it consistent.
The main difference between my work flow and yours is when you automatically clear bookmarks. I found in practice automatically deleted when only 20 seconds are left resulted in them almost never being deleted automatically. A lot of my podcasts have a commercial at the end where I would stop listening. I also a lot of time would get to my destination with a few minutes left and just not care enough to only listen to the last little bit next time. That sort of thing is a personal preference obviously but I haven't seen any complaints about this setup so far. I'm curious what other's opinions on this are.
I also really like how in DSub when you go to play an album it asks you if you want to resume or start over. For something like Audio Books you probably don't want to start over, but I also use bookmarks for keeping track of my spot in albums a lot. A lot of times the first time I go through an album I am looking for the best songs to add to playlists so I want to know whether I finished the process or not. This is once again a very personal preference.