I've just seen two more pictures from 1971. "Senior" doesn't look very well in any of them.
You're right: there is something bizarre about the father's posture or position in the picture of the three of them around the Christmas tree. He's laid out or arranged in a very strange way.
I don't know where the picture comes from. So who knows? It could be a forgery. There are certainly enough of those around. But if you were going to fake a picture would you make it that obvious?
I've gotten distrustful of throwing around terms like "parallax analysis." People who claimed to be sophisticated analysts of the last document didn't have a clue about fountain pens and mechanical typewriters, so I'm skeptical about claims of sophisticated technical analysis. People may just be discovering that old technologies don't provide the kind of clear images that we are used to now.
FWIW, It does look like there's something wrong with the figure of Obama Senior in the picture you referred to. It also looks like the print is pretty old and that there was something wrong with the print or the camera or the scanning. Notice the tiny white triangle in the upper right hand corner and the discolored area at the top center: the scan may have been off center and the print could have been bent and not in very good condition. That doesn't account for the father's posture, but it could explain part of why the picture looks so strange.
If Sr. got his stay here lengthened by marrying and if he was going to be within a few thousand miles of his ex-wife, it wouldn't have been out of place or unthinkable for him to visit her and the child for a day or two. A complicated program of forgeries that aren't very convincing wouldn't be a more likely scenario.