In regards to the ask about Harrenhal how legal is Cersei seizing Rosby on the death of its childless lord and planning to give it to one of her vassals? Isn’t that similar to what happened to Harrenhal? What is the difference between the situations exactly?

Of all the things Cersei does in AFFC, this one is actually kosher. As both King and overlord of the Crownlands, Tommen has pretty broad powers here, because without an heir, Rosby is technically in escheat, and has reverted to the crown. 

What makes Harrenhal unusual is that, as a part of the Riverlands, normally it would be the direct overlord of the Riverlands (the Lord Paramount) who would exercise those rights, but because of the unique way that Aegon the Conqueror dealt with Harrenhal, the Crown exercises direct authority despite the subinfeudation that makes the Lord of Harrenhal a vassal of the Lord Paramount of the Riverlands first, and a vassal of the King second.