As far as the original question is concerned, should girls daven, I think that should be left to the parent to decide and manage, in conjunction with the girl’s Yeshiva, and their Posek, if they wish to consult one. It is certainly not the business of any visitor who goes to someone’s house for Shabbos. I don’t believe you have any business “gently questioning” their mothers. If you did that in my house, while you were a Shabbos guest, I would gently tell you that it is none of your business. I certainly wouldn’t ask you if you davened or not.
Why are people always so concerned about what others do? Are you so perfect in your actions, that you have nothing to improve upon, and you can start on your neighbors and friends? Why don’t you just live and let live. If we all did that, this world would be so much better.