How did Heathcliff manage to force his son's wedding to Cathy? If I understand correctly, wedding banns would normally need to be read out in Cathy's church for three weeks before the wedding could take place. If anyone objected, the wedding could not take place. In Tess of the d'Urbervilles and Jane Eyre, the bridegroom avoided this process by applying for a marriage licence, but Linton Heathcliff is under the age of majority, so could he have done this? Surely it would have been highly unusual for a sixteen or seventeen-year-old woman to undergo a wedding without her parents' consent, especially when Cathy's father was a magistrate, and wealthy and well known member of the local establishment. In addition, the priest would surely ask Cathy whether she was marrying Linton of her own free will.
Also, did Linton and Cathy ever consummate their marriage? Linton was so ill at the time, he may not have been up to it. If they never consummated the marriage then it could have been annulled, and neither Linton nor his father would get their hands on her property.
This seems a real weak point in the plot.