All these problems with upgrading PCManFM seem to have
been the result of not recognizing the need to upgrade
LXDE components at the same time.

Synaptic doesn't tell you so, but you will find any
element that begins with 'lx' ought to be upgraded
as well as PCManFM to get the whole system to work right.

This includes at least lxinput, lxsession, lxterminal
and lxpanel.

Open up synaptic to ALL repos to make sure you capture
all the possible 'lx...' elements capable of being
upgraded.

PCManFM-upgraded works really well when it has all the right
LXDE supporting structure upgraded along with it.