I agree with all of your observations, but I am looking at the legal fact of the bigamous marriage to which BHO Sr. agreed, doubtless knowing fully well she was pregnant. To me the implication is that it was probably either a “shotgun” affair for BHO Sr., or it was arranged to cover for FM Davis either for money or on possible orders from the KGB.
In any case when a guy marries a four month pregnant 18-year-old, it is an admission of paternity by the man, at least to the state department of vital records. And the admission by such a marriage knowingly seals legal paternity of the child born of that pregnancy as being the child of BHO Sr.
As a result of his agreement to marry SADO on an out-of-the-way island out of sight, he was able to remain single in public. BHO Sr. may not have known that the U of HI was feeding all of this incriminating information about his playboy ways and his marriage to the INS and thought it might make him look bad and get him deported if admitted fathering a son with an underage girl to the INS while married back in in Kenya. He was right, because that is what eventually got him kicked out of the country.
The doctrine regarding paternity of children born during a marriage to which you refer is legally a presumption, not an admission. These are two different concepts under the law. Marrying a pregnant woman is not technically an admission of paternity.
I happen to agree with you that the most plausible explanation is that he likely did marry her (or agree to pretend to be married) for the reason that at that time he was under the impression he was the father.
I think he came to suspect otherwise and thereafter did his best to ignore her and the child.