Alright, thanks for clarifying. I can have Great Grandpa ride a TNT-carrying motorcycle instead. Not really planning on using explosives much aside from him: as you pointed out, I figured they won't have too much TNT, but I want to set a precedent for how even the most unlikely rebel can make a difference. Still unclear on what is the expected post quantity, but I suppose I can make a paragraph or so per post.
On that note, for second rebel post, and first in the magic family, I was thinking of Great Grandma Henriksson coming in and casting Summon Bee Hive. This would result in conjuring a bee hive, angry bees included. Bioshock portrays the power super OP, but I imagine that while distracting and painful, it's not super deadly unless a guard has allergies so it should be fine for the low-power rule.