Subject: Re: SCOTUS finds immunity for offiical Pres acts
A POTUS could argue that only a POTUS could order that, and therefore it was an official act. I don't know how posse comitatus would come into play, since SEAL teams (or any military arm) aren't supposed to conduct operations (outside of training) on US soil.

He could argue it, but he'd be clearly wrong. Trivially because there are other officers in the chain of command who can (and do) give orders to Seal Team 6. More importantly, because the fact that the POTUS gives the order doesn't make it an official act exercising executive authority. If POTUS orders Seal Team 6 to pick up Tiffany's dry cleaning or man the phone banks at his re-election campaign, that's not part of the executive function under the Constitution (nor is it an authorized power). So it wouldn't be an official act, and neither would a political assassination of a U.S. citizen on U.S. soil.