I haven’t seen too much on Yowell in Mexico, but if I happen upon anything I’ll let you know. It’s probable that Yowell followed Shelby as far as Mexico City and then disbanded with the rest of the men and then took the construction job. It’s unlikely that he continued to serve with Shelby after reaching Mexico City, as everyone left in the command, including Shelby, basically went their own way. Unfortunately Davis’ “Fallen Guidon” was based primarily on the writings of Shelby’s adjutant, who tended to be very vague on specifics and often makes it unclear as to what actually happened at times.
Thanks again for your help!