Idea at bottom of loooong paragraphs, that I think he might be in covert military.
I dug into this one on newspaper dot com and ancestry dot com. His father's obituary in the Racine Journal Times on 12/25/1986 lists Frank as a surviving son in Racine. His mother's obituary in the Racine Journal Times on 12/28/1992 lists Frank as a surviving son in Racine. I can find engagement and wedding announcements for his siblings, but not for him. His father had an older brother also named Frank Thomas Christensen, and he died in Racine in 1983. So it's possible that missing Frank went by a different name to avoid confusion with his uncle's name.
Per that article above about him being a mail carrier, on 6/3/1973, they said he is 14 (the address aligns with his parent's address that I found). They said he was in 8th grade, but I believe he had just finished 8th grade. That puts his year of birth around 1959. On 4/19/1960 in the Racine Journal Times, there a birth announcement for his parent's having a girl. So he was not born in 1960. On 9/4/1975 there's a article that says he is a former Horlick High School student, had enlisted in the Air Force and would take law enforcement specialist training after completing basic at Lackland AFB, San Antonio. That implies that he attended Horlick High School in spring of 1975. The age to enter the military with parental consent was 17. So if he was 14 on 6/3/1973, but 17 by at least 9/4/1975, then he must've turned 15 very shortly after the 6/3/1973 news article. OR his parent's helped him fudge his birth date for entering the military. Either way, it seems he did not complete high school.
Incidentally, there is a Thomas Christensen in the 1975 Jerome I Case High School, who looks EXACTLY like him, glasses and all, but not sure how that could be him.
I cannot confirm in any way that he married a Melissa or had a child Charity.
I have to say, I'm starting to suspect that he did not actually go AWOL, and is not actually missing. A possibility is that he went into a classified section of the military, and that's why the records are so very scrubbed. The reference to military 'law enforcement specialist training' is what makes me think this. The loss of a spouse and child, could've made it so he decided to take deeply undercover positions.