Do I need window trim for a chair rail
Can I answer your second question first? If other windows in the vicinity are trimmed, then you should probably trim these windows. If you do, you want to take styling cues from everything around, but without that information, I'd imagine that something like a simple fireplace surround with 2 extra pilasters would be nice. (Comment back if that description made no sense and I'll mock a picture.)

If you don't trim the windows, I'd put bits of chair rail up with tape and see what you think. If you like them, go for it.