By the fact her hair is the wrong color to be Rin. Otherwise I was thinking that her face looked like Rin. Still doesn't explain what's going on in that comic.
Considering how often Type Moon makes some new character with a character design that's basically just reusing an old one, it wouldn't be that odd if there was some minor character that was a Rin-lookalike maid.
The only time I've seen Blue Archer colors is in The Hill of Swords. That said, having him as an inverse of EMIYA's red robe and white hair actually looks really good.
But then again I'm a sucker for white over black outfit colorations.
I think it's a JoJo pose. Really, it's just a case of it having reached meme status at this point, probably. Heck, Type Moon even used that pose for Hakuno's sprite in Servant Capsule.
