Why would all these specific puritan splinters want to be turned in religious minorities, which would cause them to lose social and political power.
They were
already religious minorities (in the sense that no one of them could claim to form a majority or even a plurality of their colonies) that were shrinking for autocthonic reasons, quite aside from any immigration. In the run-up to the American Revolution, many non-Puritan religious groups such as Anglicans were already growing into quite substantial demographic groups in New England, while the Puritan-derived groups were being eaten at from below by what were effectively new religious movements like the Baptists. This is essentially baked-in by the 1760s and 1770s.
That was to set a consistant standard of naturalisation. It was still up to the colonies on whether to accept migration.
But this effectively takes away the ability of the colonies to decide that, for exactly the same reason that federal control over naturalization did. Even if Massachusetts decides not to accept migrants, New York can, and then naturalized citizens of New York can legally migrate to Boston and there's nothing at all that Boston itself can do to stop them without breaking the fundamental principle that the colonies allow any British citizen to reside there.
Albany and Galloway plan would just unite American interests against British interests, which wouldn't be in Britain's favor, nor would colonies want to give up their identities for no reason.
A unified government would hardly mean "giving up their identities," much less doing it for "no reason" given the utility of a common governance structure to address common problems to the colonies. It is significant that the idea of a unified government repeatedly arose between the Seven Years War and the Revolutionary War and gained more and more support over time until finally the United States formed. Clearly there was a perception--small at first but growing over time--that common institutions were needed to solve common problems.
As for the British, it is certainly much more in their interests to have America unified but in their general sphere and subject to Britain in a loose way than to have them independent and not necessarily friendly. Any kind of resolution of the British-American conflicts between the Seven Years War and the Revolutionary War needs to find some kind of
via media between total independence and complete subjugation to the British Parliament, since the latter was unacceptable to the colonists and would be extremely difficult to practically enforce and the former, of course, is simply OTL. That probably means some degree of consolidation of colonial governments, even if not necessarily into a
single government, so that the colonies are more subject to representatives from the colonies than to members of Parliament in London who have never even been near America. If Parliament is not able to recognize this then it will not be able to avoid a Revolutionary War.
In any case, I was merely pointing out that this had been advanced as a proposal and your comments did not address it at all.
Well Australasia wasn't considering it's lower amount of migration, nor was large amounts of the US. Labour shortages generally just mean industrialists want cheap labour.
Yes, and? Industrialists are politically powerful and have large influence over policy. Additionally, there's a difference between a labor shortage and a
labor shortage; given the land area of the future United States, there were just not enough people to go around to fulfill many tasks, not just industrial ones, which was something that was rightly noted in many colonies before the Revolutionary War and addressed partially through attempts to import many immigrants. Australia, by contrast, is largely desert and considerably less suited for dense settlement, in addition to being much farther from Europe. It is not a very good point of comparison. By comparison Canada attracted a large number of immigrants from both British and non-British backgrounds.
How is London going to exert power over an autonmous community that want's to be left alone, such as Statutory Neglect.
Presumably through the fact that they are the supreme law-making body in the Empire and control the courts and the army, just the same way that the federal government did IOTL. In practice, the Revolutionary War stemmed from a collision between Parliamentary attempts to increase central control over the colonies and make them pay more in revenue and colonial desires to be left alone to direct their own policy. As I said, some type of middle ground needs to be found between total subjugation and total independence. While this might mean that London has limited power over this "autonomous community," it will definitely have
some power (or we've just gone back to OTL and the whole exercise was pointless).
There is also the fundemantal issue of, a majority non-British empire would not want to be part of the British empire. Quebec or the Boers did not want to be under British rule.
And yet Quebec was actually rather placid through most of Canada's history as part of the British Empire. And you are assuming that immigrants will not assimilate into the local population (as happened historically in Canada) and that attracting groups such as the Doukhobors, Mennonites, and Hutterites for strategic reasons to frontier areas means a "majority non-British" empire somehow.