No one can positively tell you with a maiden but her nipples are still flat but maidens sometimes are. I know mare stare is hard but with maidens it can be anytime hopefully from 300 days - 350 days. You have said she has little tail resistance but her sides still look to be wide she should drop lower and the sides will almost disappear. JMO you have some more time to wait.
Yes, with maiden mares it is hard to tell sometimes. My maiden mare (first one) foaled with NO BAG whatsover and even after she foaled she did not get a bag but did on her foaling date.
Typically the nipples will fill with milk and become fuller and also point straight down and out a bit. I just had a mare the other day who had a rather firm but not big bag and no sticky milk at all on Monday night. Tuesday night I checked her and her bag was tighter (but not bigger) and her milk sticky immediately. She foaled the next night. Some mares/maidens are text book.
We have a name we call this: Mare Stare, Foal Watch etc., most of us cannot tell exactly when they will foal. Not even the vets can tell you. There is a wide range of when they can foal. Given your dates though it could be anytime as the baby can move up into the birthing canal in a very short period of time, this causing the bag to change, milk to fill the teats. The V will disappear as well just prior to foaling as foal moves upward into position.
Good luck, please show us your photos when the baby arrives!
PS: I am no expert, just my observations of foaling over a period of years.