Sliding/nudging to the LEFT does not work...??

It usually has to do with your snap settings. Like if you're set to Move By a measure but the beginning of the clip is on the second beat of the first measure, it can't move by a measure because there's only two beats available to the left of the clip. Also if you select all with ctrl+a and try to move a bunch of clips to the left you can't because the selection extends to the beginning of the project.

I think slide will only move clip automation, not track automation, and with clip automation you're stuck with gain envelopes and not volume. Nudge will move track automation, though, so you should probably try to get that working. Make sure you didn't accidentally turn off the NumLock key, and also check your keybindings. And you have the option of setting your snap to absolute and dragging clips when you're zoomed in and maybe adding another ruler with the appropriate time format.