To sort of address #3:
Limbo's gimmick is that when you dash, are targeted by his first ability, or are within the zone created by his 4th ability, you are unable to interact with things outside. Friendly warframes within the zone can affect enemies with their abilities, but your bullets will pass harmlessly through enemies unless they are Banished as well.
This inability to interact also includes locked doors, consoles, and items you need to pick up, such as the data terminals for Mobile Defense missions. So if you use his 4 to surround a mobile defense target, you will not be able to activate the target until the bubble collapses. This is prone to griefing, as it prevents your teammates from accessing the target, but it is also useful, as enemies will not be able to damage the terminal until they walk inside the bubble.
This combines well with his Stasis ability, which freezes enemies who are Banished. So a player might cast the Stasis ability then place his ability 4 bubble over a target that needs protecting. Enemies will be unable to damage the target from range, and if they walk into the bubble they count as banished, and will be frozen by Stasis.