1: The deploy bonuses affect all the units in your team, regardless of their placement in attacking the enemy or defending your position.
2: An unit with Defense Order will affect all of your units, while the IW unit will only defend itself. So, if you're using 5-man defense teams, having a Def. Order unit will help more than an aditional IW (take Rin).
3: No, unless you decide to change your strategy. (like going for offensive attacks or defending your base).
4: Having Incinerate units is a good choice in Guild events. So, if Lavinia's your only Incinerate unit, taking her is advised, as she's a good choice when speaking about 20 DC 6* units.
Hope i helped on your search for anwsers. As you play more often, you'll learn about your capabilities. Take every victory and defeat as a chance to learn about your team, what it needs to be better (never perfect, as you'll find spenders that will crush you regardless of skill).