#1
For example:

7 7 6 6
- - - -
9 9 8 8

What if there is no "X" in the tablature. How would you play that?
#3
You could still mute it if you want, doesn't realllly matter. That or you could use your thumb and index finger to finger pick it.
#4
I usually just mute sections like that lightly with one of my fingers.
#5
Play it like there's an X there anyways, it's not like you can hear it when you play, especially if it's a rock or metal song.