Alex, this looks like the correct output for the debug code. You can see that in the first set of data you have before the session is destroyed there are cart contents including the items and the column values for them.
After destroying the session the only thing that is left are the user variables you set back to session variables. The warnings you are getting about the regenerate id should go away if there is no debug output occurring.
Do you have a link where this can be tested out? Is there a certain set of steps that needs to be taken to see the bad result? Please post back with this info so we can look into this further. Rite now it seems that the session is being cleared properly when this page is reached. If you can't post back with this info let me know and I can get a ticket opened up for you.