When determining if the character has 11 stars or more, does it matter which mode it’s in?
[Source: WotC]
No. A character always has the number of stars printed on it, no matter which mode it’s in.
If you have Energy Pack on an 11-star character who is nearly dead, then attach
Erratic Energy Grenade, removing Energy Pack, which KO’s your character automatically since their health bonus is no longer there, does Erratic Energy Grenade’s second effect still activate?
[Source: WotC]
Yes, Erratic Energy’s Grenade’s second effect would still activate in this scenario.
Erratic Energy Grenade replaces Energy Pack without anything happening in between.
Energy Pack reads "The upgraded character has +4 health." When I play this on a character does the character get an immediate 4 to its health? Or does it just increase the health total?
[Source: WotC]
Gaining +4 health has no effect on the amount of damage counters a character has. For example, if you had a character with 10 health and 3 damage counters, upgrading it with Energy Pack results in it having 14 health and 3 damage counters.
I’m playing with a plane card, for example Captain Starscream, and it has on it the utility card Energy Pack. The total damage he has on him at the time is 19 and then I play the utility card
Crash Landing on him, scrapping the Energy Pack and instantly KO’ing him, does the Crash Landing take effect or not?
[Source: WotC]
No,
Crash Landing only triggers on your opponent's turn, so it would not trigger here.