The title is very misleading. This just talks about Earth coverage of missile launch detection system according to publicly available information. This has nothing to do with detecting submarines.
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.
Both RF and US operate FBM forces as deterrent second-strike forces. This is not only the operational reality but also the treaty-regulated deployed configuration.
Optimizing deployment in the manner you describe is not aligned to the current mission of these forces / our deterrence relationship.
That's not necessarily true anymore. The Russians have deployed the Sineva and the Layner SLBMs which have GLONASS supplemental guidance. This makes no sense in a second strike scenario. The Trident II is also plenty accurate for a counterforce first strike and allows less reaction time than ICBMs.