Earlier quoted context omitted.
presumably the reason you would have IR satellites looking for launch plumes in the middle of the ocean is in fact to detect submarine-based launches
The deployment of submarines is rather orthogonal to the coverage by early detection warnings. Detection is not helpful if the flight time of missile is less then reaction time. So one wants to deploy close to US coast line irrespective of detection coverage. Besides, launching from South Pacific means US will have plenty of time to detect the warhead with normal radars.
Optimizing deployment in the manner you describe is not aligned to the current mission of these forces / our deterrence relationship.