Comment 82 for bug 1251281

gnome-session was aborting if acceleration check failed. This bug was about fixing it so it is possible to bypass this check. Bypassing this check allows gnome-session to start.

You need to use session that does not require hardware acceleration, but from you comment - you are using. As Ryan already told - you need to get and paste log file to find why session does not fully start.

lightdm and other display manager are setting environment variables - this might be your problem.

One thing you could try is to add this line before /usr/bin/gnome-session line:

Then you will need also this line:
export XDG_MENU_PREFIX="gnome-flashback-"

Also you could add --debug to gnome-session line as it might show some info why needed components are not started.