“My understanding is that the law does indeed (or did indeed) distinguish critically between married and unmarried mothers, and impose different residence rules. “
Only if he was a US citizen, which Sr wasn’t. There are specific rules for those whose parents are of American citizen and father of foreign citizen.
Only if who was a U.S. citizen? The father? The father doesn't count, if the mother is unmarried. That's what I understood.
The father might as well be a phial of refrigerated, unprovenanced sperm, if the mother is single. I don't understand why you keep trying to drag that rolling stone into it. He was never in the picture legally, because his "marriage" to Obama's mother was not legit. No marriage, no fatherhood. No fatherhood, no impact on Obama's citizenship status.