My understanding is that Hezbollah is just a proxy of the Revolutionary Guards. Even if not, they share many of the same goals and objectives. Its not in their interest to see Iran's government destroyed. I don't think Hezbollah would sit on the fence during a large scale engagement between the U.S. and Iran.

As for the cells it doesn't take much to change direction from simple fund raising to a more active posture. At least several of the members had combat training in Lebanon. It makes sense to think if they had the foresight to infiltrate fund raising cells that they would also have other sleeper cells acting more covertly.