"How much of Britain's vaunted 17th and 18th century navy was designed to protect the commercial looting of the colonies? "

I won't venture commenting on the rest of your post, that's a minefield I will save for some other time. However, I would like to make some remarks on the statement you made, quoted above.

Although there is no doubt the Royal Navy supported and expanded the Crown's commercial interests in America (as well as Africa, Asia, Europe and the Mediterranean) it wan't originally designed for that purpose. THe British Navy originated as a pirate force, privateers commissioned to prey on Spanish shipping when that nation was the world's leading naval power, during the Armada days.

By the 1600s, Britain's now professional Navy found itself in a long set of wars with Continental powers, primarily the Dutch and the French, which could be traced to colonial rivalries with those nations. The 18th century (and Lord Nelson's) Navy's primary purpose was to blockade Europe and deny the French use of the seas. It was used both offensively (to restrict French trade and transport and to support British operations on the Continent) and defensively (to frustrate Napoleon's plans for invasion of the British Isles). During the 19th century, British Naval power was brought to bear to protect British trade and commercial interests, and to patrol her colonial Empire.

Even in the days of fighting sail, the social and financial costs of supporting such a huge and highly capable force (at one time the Brits had a hundred ships of the line, and numerous frigates, sloops and support vessels, and they were superb seamen) was part of the operating expenses of maintaining a colonial empire (one which the great commercial houses did not have to finance out of their own pockets). In a way, these commercial interests were not only made possible by the Royal Navy, they made the Navy possible. The two fed off each other, each justifying the other.

It can be argued that the Navy allowed Britain to fight both World Wars on foreign soil, so they paid for themselves in the end, but it is also equally true that maintaining this force had already broken the back of the British economy, and that even the private income from the colonies
did not equal the public expense needed to protect and administer the Empire. By the turn of the 20th century, the United States was already the world's leading industrial power. It's no surprise that around this time the US was already assembling its own Navy, and its own empire.