I haven't had a chance to read all the posts unfortunatly, but I think that Captain Tilney had another motive for leaving his family and joining the military.

Mr. Tilney, the former General, clearly valued work and industry over idleness and dissipation. I've always thought that Fredick's career in the army was mostly the result of Mr. Tilney's insistance. He could not bear for his son to be like other sons. Many, perhaps most young men in Fredicks shoes who were to inheret a great fortune did not join the army but spent their time hunting and the like, like Tom Bertram.