I think installing compiz will have stopped those keyboard shortcuts from working, and you may need to change settings with CCSM to get them going again.
Even in gnome the Alt+F2 shortcut will not work if compiz is the WM, unless you set the shortcut in the Gnome Compatibility. Unfortunately I don't know how you do the same in LXDE, but you could try setting those in the Command tab of that plugin, if it is present, and see if it works.
I also have a problem sometimes with mounting usb flash drives on an old laptop, but mine is definitely a hardware problem as the drives are not recognised at all by the system; even dmesg does not respond in any way. I think your problem is the ext4 filesystem needing appropriate permissions to mount the partition as user; I expect you can mount them as root if you try. If you label those ext4 partitions with tune2fs commands
they will always mount to /media/label, with those labels as the folder name and you can then set ownership as needed for those folders with
sudo tune2fs -L <labelname> /dev/sdx#
and permissions if you need to with
sudo chown user:user /media/label
choosing the 7xx as you want. The deafult for /home in Ubuntu is 755, so that may be the most appropriate for that folder as well.
sudo chmod 7xx /media/label