p.s. Thank you for warning about direct references to \Fuel\Core classes. I use direct references to \Fuel\Core classes instead aliased version because Eclipse doesn't autocomplete aliased version. I can't write code without autocompleting.
As you have written down: from 1.8.0.3 to 1.9/develop there are no changes which could affect sessions. I added every single change manually and session was not affected. But dev-1.9/develop version is OK for me.