Hi Norrie G,
Perhaps you have already seen it, but the Cupar library has an article on his death in Galveston, dated 6 Mar 1891; East Fife Record.
Also on Ancestry.Co.Uk is a family tree posted for David George Guthrie who fits and is worth study.
Exhaustive studies of the 1880 Texas census did not reveal anything conclusive except for a Capt D Guthrie (Page 11, dist 62) b 1832 Scotland married to a "D Guthrie" of Hamburg, Germany. Doesn't really fit.
None of these answer your questions, but you did get some effort and this might help.
Joel