Okay so I figure I should make public the details of why I wanted to ask Flan, and why I thought it worth doing.
The first why is easy: Flan is cute and funny. She can be Nue's little wingsis sidekick and be great for entertainment value, if not a quality extra body should things go south (no pun intended). The second why has a few reasons - Flan wants to help, but Remi got mad at her and there was almost a fight. It's pretty obvious that Flan would be in the way of research because Flan's a doer, not a thinker, which is most likely why Sakuya sent her outside to the gate. Taking her with us, however, gives her something that can be an active help job while not removing her from an important task (since Meiling can probably do the gate by herself). This keeps Flan happy and lets her help while allowing Sakuya and Remilia (and Patchouli, for all they know) to go about their business uninterrupted. And, of course, there would be no alcohol, no flirting, and no asking Flan to fight unless absolutely necessary, all of which I am sure would be three of Remilia's immediate concerns when presented with the idea given who we are. (Having it be nighttime now also alleviates the sun issue.)