I only know the gelding part from first hand knowledge. My last horse was a studdy gelding. He loved the ladies! He can and would mount every single time the mares were in heat. Not just once either! After pressure from the mare owners, I had him blood tested for testosterone levels. He is a gelding...for sure. We know that he was cut late and that has a lot to do with his behavior. In our situation, he did end up being fielded with a mare...just not one who was terribly receptive to his come ons. No matter if she was in heat or not.
My advice is, if his sheath is cleaned and he isnt terribly agressive...let it be. I highly doubt your mare is pregnant. It would have to be a miracle of miracles.
Good luck! I know these situations are frustrating...its just animals being animals