Could a widow like Lady Hornwood plead to their liege lord to not remarry? If they were being forced to marry could they seek help from their liege lord?

They could, and the lord might agree, because that might set up a scenario down the line where they make use of their right of escheat.

Or the lord might decide that they want to give that land to someone, and try to make the widow marry someone, or just turf out the widow altogther.

Depends on what the law was on widow’s rights, the deceased’s will, and so on.