I do not believe that there is anyone here who can answer 100% your first & second question.
But...
1. It depends on how your team is composed and what to you want, also how you fight. Light buff your party and Dark Curse the enemy's party. I prefer a party with both light and dark mystics... so if you chose one of them hire a merc with the oposite skills... it will be like double effect... (But remember the dark side may consume you, plus we all now that the bad guy lose at the end of the game, or movie
)
2. Again every person have his own compination. My opinion is:
A) for light ->Divine Bless & Heal Mastery (my party is 3 (i am one of them) meat tanks and 2 damagers + 1 mage)
B) for dark ->Sickness & Petrify or Losing Focus.
3. to all buffs
4. Full (but not 100% sure
)
5. as you can see here
http://forum.imperialhero.org/int/index.php?topic=499.15 Divine Bless
Grants bonus to all party members' attributes. Last level grants mass effect.
But according to posts (reports) they work different. Tiger's Strength is used every rount untill all members are buffed with at the last level you can do that in one turn.
Divine bless add the same bonus and every level adds a team member since it is used only once at the start of the battle.