On YouTube it is very frustarting that they override basically every system level hot key. I.e. cmd+l to go to the search bar, cmd+1 to 6 to switch tabs etc.
Is there anyway I could use Greasemonkey to disable these actions?
I believe the YouTube shortcuts you mention are only active whilst the video itself has focus. So you can simply click off the video element, on another part of the page, to effectively disable the keyboard shortcuts. These shortcuts would seem to be part of the Flash player, not the HTML page.
I don't think there is any other, more permanent, way of disabling these shortcuts.