Regarding whether David Guitar's membership in the EMM indicated a recognition of the importance of serving--membership in the EMM was required of all loyal civilians. Now he would have had to have voluntarily had his name put up for nomination as captain of his company since company grade officers were elected in Missouri. Such might indicate a recognition of importance, but not mere membership in and of itself.
As I noted in my original post that started this thread, and which you reiterate in your own analysis, David Guitar's post-war activities and affiliations unambiguously tell us he was recognized as being loyal. It might further be noted that his business partner, John F. Baker, appears to have been a Radical Unionist--he was appointed sheriff of Boone County by Radical Governor Thomas Fletcher in 1865 (Baker had previously been an officer in the Unionist Columbia Tigers Militia--the namesake of the University of Missouri mascot). This is not dispositive of political sympathies by any means, but the Radicals had seized complete control of the reins of power in Missouri in 1865, and they were wielding them quite aggressively.
Regarding whether or not Odon Guitar was a slaveholder--perhaps not at the time of the 1860 census, but it is reported that "In May 1861 a Union meeting was held in Columbia. In what many witnesses reported to be his finest speech, Guitar stated that though he had owned slaves and had been taught that slavery was of divine origin, he would gladly give up any and all benefits of it in order to preserve the Union." This speech reportedly resulted in George Caleb Bingham writing Congressman James Sidney Rollins and stating "Guitar is the truest man you have among you, all honor to him." I do not have a primary source for this information--it comes from the Odon Guitar bio in the 1999 "Dictionary of Missouri Biography." The bibli for this entry includes the 1916 "Bench and Bar of Boone County," a 1928 article on Guitar in the Missouri Historical Review, and two 1970s-era master's thesis (including one by a Nancy Guitar). Possibly the report came from an 1861 newspaper article that was cited in one of the master's theses. As we have seen the folly in taking another's research at face value, if I were writing a story on Odon Guitar as it relates to his slave ownership, I would hunt down the primary source rather than relying on this 1999 secondary source.
The bottom line for me in all of this David Guitar contretemps was the newspaper report from a little over a year ago on the David Guitar House stating that David was a southern sympathizer who had built the house in 1862 (go review the very first post in this thread). This late 2010 newspaper report immediately struck me as stating an impossibility--southern sympathizers weren't dumping fortunes into building mansions in Boone County in 1862. They were being taken out and shot and their homes were being burned.