Can I take passion flower if I am taking remeron 30mg and Xanax 0.5mg at bedtime? What other herb might help anxiety through day? Or perhaps buspar (buspirone)?

Passion flower. I would not recommend taking passion flower at the same time as other sedating medications (remeron, xanax) as it could increase this effect. There have been only a couple of pilot studies of passion flower extract, rather than head-to-head comparisons to pharmaceuticals. Passion flower could be taken during the day, or even bach rescue remedy. Buspar (buspirone) is reasonable also, & is more studied.
BuSpar (buspirone) Might. The evidence for passion flower is quite weak. And in general there is limited evidence for the effectiveness of any natural supplement for anxiety reduction. Buspar (buspirone) is more likely to be helpful but it takes a while to work and because it is not at all sedating (like xanax) it is often hard to see the benefits. If you try it be sure to do an anxiety rating before and then 4 weeks after.