I also don’t think Maurice would hurt her–at least intentionally. He seemed quite unwilling to let Rumpelstiltskin take Belle–he ordered Rumpelstiltskin out of his castle, and then later begged Belle not to go with him. He seemed very saddened by the deal. From the way they talked to each other, they seemed very loving and close. It would seem out of character for him to suddenly shun her and treat her with cruel behavior.
1). Belle never made it back to the castle. No doubt she may have slowed down to travel, but I think she would have eventually returned to her father to ease his worry after she was done with her journey. But on her way back to her father’s, or during her time seeing the world, she was captured by the Queen and imprisoned.
2). Belle never got captured, and was simply locked away as part of her unhappy ending. This could also be a possibility, though doubtful. The Queen would probably want to capture Belle quickly, just in case Rumpelstiltskin decided to investigate the story. But she may have by then known that Rumpelstiltskin was working on the curse, so she may not have been in any hurry.
3). The Queen may have convinced Maurice to give her Belle. It is not hard to picture the Queen coming to Maurice and spinning a wild tale about Belle being corrupted, and the the Queen was the only one who could help her. And I could see Maurice believing the Queen–she is very good at tricking people, and manipulation.