It's great you're trying the passenger lists...I'll do the same tonight at the library. I hadn't considered the "twins" hypothesis for Luke G. and Thomas, but they certainly might have been. In my other posts, I gave information concerning the Palatine (Cook Co., IL) land purchases made by Luke in 1844. Luke and Thomas clearly were very close to one another. Perhaps Luke purchased the section 33 properties "in name only" and Thomas lived on the land. Just a guess, but that way the family "black cloth" story and Luke's travels makes sense.
Nice work, too, on the 1870 census and Thomas and Lavina's kids. I'm sure she died in Illinois (or Eureka, Greenwood, Kansas) between 1865 and 1870. Her boys are scattered by 1870. It's too bad we haven't found Dewitt E. Torrance in the 1870. Lavina might have been with him, although I've tried multiple search techniques and can't find a woman of her description in the 1870 census.