I only use live cover with my stallion and he has never attempted to mount a mare when she was not in heat.
He also does not approach any mare from the rear in order to avoid being kicked.
Biting the mare on her flank or hip is what stallions do to test and tease the mare.
He has never left a mark when biting a mare and she always accepts him without a fuss.
If they do not think she has come into heat why are you picking her up? Mares left here for breeding are covered when they are in heat and stay for the next cycle. 14-21 days after breeding we have the mare owner US that mare.
Sounds like a young stallion and inexperienced breeders.
What does your breeding contract state?
I also question taking a mare and putting her in a pasture with a stallion 3 weeks before she is due to cycle. Shalom