I honestly don't think we can be 100% certain who it is until we get some Canon confirmation. I don't think we have enough information, and the information we do have is being interpreted a bazillion different way.
I have no idea about the appearances and voices, so I'll speak simply to the behavior of the character. I think Alex is ambiguous enough to give information to TAAG behind the Order's back, protecting her identity. Nikki B would be awesome, is smart enough to get this information, and although it may be uncharacteristically serious for her, I don't think it's that unbelievable given the situation. As for Beth, we don't know that much about her, so we can't rule her out either way, I think.
Like I said, I have no idea so I can only speculate: My theory is that it's Alex. My hope is that it's Nikki B. Yes, it seems increasingly unlikely, but I'm Optimistic to the point of Delusion.

blendedchaitea, you are so awesome to go check out the location. (The idea of a drop is so exciting to me.) But I know none of us want you to do something dangerous, so please be sure to take every precaution. And be sure to report back ASAP!
