[SaS TrooP]

1) Not really, terrain shape is what limits your LOS. Beware of terrain such as high grass, that has sudden height changes. Keep in mind that clear forests allow to see even 2-3 hexes into them.
2) Could be. AI resigns from returning fire if chances are obviously very low or they cannot see the shooter
3) Usually. Yes. If its cover, not concealment. There is a good sheet showing cover efficiency on various terrain
4) AFAIK yes. It can also loose target when firing at different unit while making opportunity fire
5) Suppression rises slower in cover. But it's a bit sloopy sometimes

"T" button for target, then "N" or "P" for next and previous visible target.

Tank riders can ride a tank if tank's carry capacity is same or greater than carry cost of tank rider unit. Select bigger tanks, that should do the trick.