May a woman serve as the sole kvater (person who brings the baby to the sandek) at a bris? At most britot I've been to, the duty of kvater was handled by a woman and a man (typically married), with the woman bringing the baby from the mother to the man, and the man bringing the baby to the sandek, I'm asking if a woman (or women) only can serve that role.
A woman can be a kvater(in) if the physical layout of the location of the brit allows her to do without tzniut issues.
The Rema in Shulchan Aruch YD 265:11 describes how a woman cannot be a sandak because of pritzut (licentiousness) therefore she is the kvater(in) and helps her husband by bringing the baby to the entrance of the synagogue and her husband takes the baby from her to be the sandak.
So we see that it is because of tzniut (modesty) that a woman wouldn't bring the baby to the sandak. The same issue would likely occur nowadays when the brit is performed in the men's section of a synagogue. However it would be different in a hall or home separated in two sections, with the mohel in the middle, where the woman could come to the sandak directly without being in the middle of a crowded assembly of men.
(Note that, in communities where the father is the sandak, there are issues of his wife being nidda and not being able to touch or pass objects to her husband, which is why she might require another couple to be intermediaries.)
I discussed the above with R Binyamin Tabady who concurs but, of course, one should consult the local rabbi before implementing anything you learn here to ensure he is comfortable with the proceedings.