"If it ain't broke, don't fix it."
Did the reload of lxsession from Synaptic, rebooted, nothing changed. Still manic. My laptop (this NX9005 I'm writing from because the netbook is on charge) does have a /.config/lxsession/LXDE which looks very much like yours, BTW.
Because some of my machinery doesn't have a /lxsession folder and manages just fine, as an experiment I renamed the troublesome netbook SD install /lxsession to lxsession.old and rebooted. The desktop has reverted to the Avio default theme and is perfectly stable!
With my usual caution about not knowing of that which I speak, it does seem to me that having a /lxsession is an optional extra in some installs, and in some installs can find itself thrown into conflict with another file trying to do essentially the same function. My troublesome SD behaved for months until (heresay, I can't prove it) I did a routine update which altered something. For all I know the SD didn't have a /lxsession and the update wrote it one, let's say. And conflict began. Are you able to discover if the repo lxsession has been revised in the last couple of months?
In a spare moment I will substitute your desktop.conf file for the SD one and change the /lxsession/ folder back just to see what happens there. More soon.