Uhhh...you do remember who blew up the Grail, right?
That would make it even more hilarious when the Grail finally realized what it had done. It was so focus on the younger Emiya that it kinda forgot about the older one and the rage from said realization shall be glorious.
Fair enough.
However, this might be my fault here, but I didn't try to imply that Dinah alone would be a match to Meph. That thought alone is quite ridiculous and I was more focus towards their potential relationship , which is that she'll go against Meph and definitely against in being his master. As for resource, I was more thinking of the Mayor and his connection to the PRT and other police force. Add that to the possibility of giving her a servant, she might a be a good contender for the war.
Sure, at this point she's being held at gun or bomb point and incredibly traumatized, but that's what we're here for. Giving people power, motivation, and a bit of all worlds evil.
Anyway, regarding Armsmaster, while I made the Batman comment offhandedly, I agree that he would be the more appropriate person for the reference. Still, I'm actually hoping that Armsmaster wouldn't go down the Batman path for this quest. Seeing him observing the people around him and their state, realizing how little he had been doing and his attitude toward the general situation. While it'll be fun to see him fighting all around the city, I felt it would be more satisfying to see how he change and his interaction to others.
Edit:
Edit: I vastly prefer corrupting Dinah to having her become an antihero. Too many people who think they're the good guys. I want impressionable, desperate Masters who can be led to depravity by evil Servants.
We already have enough monster in Earth Bet, but not enough well intention extremist. All the good guys here, they have their limit. But a person who is willing to do unspeakable act of evil for the sake of the greater good, we don't have a master like that yet. For me at least, if we do get the chance to influence her, I prefer to corrupt her in that direction. A person that act like a villain, but have the mindset of a hero.