First, David Guitar's granddaughter Sarah Guitar was the long-time research librarian for the Missouri State historical society working for Society Secretary Floyd Shoemaker. She is described as the unsung hero of the society in Alan Havig's Centennial History of the State historical society of Missouri. Sarah Guitar was born in 20 Jan 1892 to James H Guitar, son of David Guitar. She was 20 years old when her grandfather died--they both lived in Columbia, so surely she knew him. Sarah wrote many papers for the Historical society and researched for many others. Sarah worked for the society for 39 years. According to Havig's book (p 53), her obituary was full of references to her luminary ancestors. Perhaps, David Guitar was referenced in the obit. Sarah died 5 November 1963. Does someone have access to her obit?
Next, Sarah Guitar's papers are at the Historic Society. http://shs.umsystem.edu/manuscripts/invent/3563.html Most pages are about her work, but a few pages are not. Perhaps, she references her grandfather. David Guitar is not indexed in the finding aide, but he may have been referred to as "my grandfather" in a personal letter and the cataloguer would not have indexed him.
Thirdly, similarly, the Historic Society has the papers of Odon Guitar, David Guitar's brother. David is not indexed, but might have been referenced as "my brother." The collection includes: Correspondence of Odon Guitar, including love letters to Kate Leonard in 1865; letters from his mother, Emily Guitar, while Odon was in the Army; correspondence of John and Emily Guitar, 1832-1846, and letter from Odon to his parents from Fort Leavenworth, 1843. Also copy of letter of J.H. Ellis criticizing Odon's conduct as provost in St. Joseph, 1863. One of Odon's letters refutes charges that Odon was an abolitionist.
Fourth. The university of Missouri has papers from William J. Burke (1902-1990) . A native Missourian, he worked for Look Magazine as a researcher for 25 years from 1943. As some may know, he also co-wrote the Missouri Waltz about Tom Pendergast. One of the folders in the collection is about the David Guitar home. It contains photographs and perhaps some notes. This material from the mid-20th century may give insight to how the "confederate" mythology evolved.
Fifth. The James S. Rollins (1812-1888) papers are at the State Historic society and also on microfilm. Rollins from a lawyer, from Columbia, in the state legislature, a candidate for governor, opposed to extending slavery to the territories, and more. Among the people indexed in this extensive collection is David Guitar. http://shs.umsystem.edu/manuscripts/invent/1026.html
Sixth. The North Todd Gentry (1866-1944) papers held at the University of Missouri. Gentry was a Columbia lawyer, judge,supreme court justice and amateur historian. Folder #177 contains information about several Guitar family members, including David and his son James Herndon Guitar. Odon Guitar is mentioned in several folders. http://shs.umsystem.edu/manuscripts/invent/0049.html
Seventh. There may be information in the obituaries of David Guitar's children--how did they refer to their father? James Herndon Guitar (d. 27 Sep 1925), William Herndon Guitar (d 30 May 1940), Martha Guitar Hale (d 1 Mar 1958 in Houston TX), Edward H. Guitar (d 16 Apr 1959)
I sent a research request to the State Historic Society for the first 6 questions. Let's see what they want to charge; I'll let you know. Perhaps, someone has access to the Columbia MO papers for the obits of the children. It would be enlightening to see how they viewed their father.