pcmanfm in LXDE is more than just a file-manager, it also handles much of the viewable desktop as well; so losing icons, the LXDE menu (on right-click of background) etc are to be expected (lxpanel runs the panel so the panel & panel menu will still work, openbox is still running so right-click on background will now show openbox menu instead of LXDE’s which needed pcmanfm).
I really have nothing further to add. If I’m unsure about a box, I tend to use it using live media until I’m sure I trust the hardware, but without response to my last comment I’ll not add more.
However please remember Lubuntu 18.04 LTS is in it’s final days of support (three years from in it’s initial launch on 27-April-2018)