Going back decades when a billion US was real money the original NSA (No Such Agency) that essentially no one had ever heard of, including most of the US houses and much of the defence committees that had clearance but not that clearance, had a 4 Billion+ budget for "off-book" satellites.
Black cables are a damn sight cheaper than black satellites.
Yes, although I think it would also clean up the visualization, since you wouldn't have nearly so many lines connecting data centers which actually aren't connected; and it would therefore also be explanatory
It's highly unlikely there are any non-disclosed undersea ones since they cost rather a lot to lay down.