Let's get to the answer.
An important technique in genealogical research is to expand the scope to multiple family members - beyond those of interest. Records of brothers, sisters, aunts, and uncles can reveal critical insights into the lives of direct ancestors. Indeed, a full picture of each family member enriches the stories of all. This proved to be the case in my search for Juan Corona.
The Corona family entry in the 1930 Mexico National Census suggests that Natividad and Juan were half-brothers. With confirmation that Cándida Uribe was the mother of Juan, and the implausibility that she was the mother of Natividad (due to their proximity in age), the next step was to identify Natividad's mother and perhaps find a clue to the relationship to the Medina family.
For Mexican research, I rely heavily on the FamilySearch database, filled with hundreds of millions of records from the Catholic Church and the Regístro Civil - some indexed and others not. Using the search tools available for the indexed records, I looked for more information about Sebastian Corona and his first marriage (children, marriage record, etc.). No luck. There simply were no indexed records in FamilySearch matching this family. I then began the old-fashioned (and incredibly time-consuming) process of searching the unindexed Civil Registration records page-by-page.
The problem with this approach was that I was using erroneous information discovered in books written about Juan Corona. Nearly all references I consulted (which largely included research conducted by lawyers, and not genealogists) suggested that the family was from Autlán. Incorrect. It appeared I had hit a brick wall.
And then, using the Ancestry.com database of Mexico to U.S. Border Crossings, I located a record for Natividad. It proved to be the wrecking ball that busted through my brick wall.
Using this new information (and, in October, the release of Ancestry.com's collection of indexed Mexican Civil Registration records), I quickly discovered the rest of the story.
Natividad Corona was born on December 30th, 1922, as indicated on the border crossing document. This was supported by the civil registration of his birth. Interestingly, he is listed as 'hijo ilegítimo' which would explain the reason for no marriage records for Sebastian and Gerónima.
The next and final step to answering the research question was to build the family tree of Gerónima Sanchez.
It didn't take long to tie into a tree I had long researched. My husband's great grandmother was Francisca Sanchez Ramos. The first record I discovered, though tragic, connected Gerónima to Sebastian Corona and to Francisca Sanchez. It was a 1927 death record for Señora Gerónima Sanchez, in which the informant was Sebastian Corona, and the parents were listed as Pascual y Basilia Ramos. Pascual and Basilia were Francisca's parents, as well.
Natividad Corona's mother, Gerónima, was my husband's great grandmother's sister. Put another way, Natividad (Juan's half-brother) was my husband's grandfather's cousin.
Eh. I'll take my chances. ;)