Reply #4: Not really.

[Igel]

Older American usage had 'insure' as a simple variant of 'ensure', and that usage survives here and there. Some style manuals prohibit it entirely, but those are people or groups of people setting themselves up to dictate usage to others. As long as this is for publications within an organization, or determining publishing-house uniform standards, that's fine. The problem is when these manuals are asserted to have general application, all the more serious a problem because the manuals do not always agree among themselves. Note that we have no royal or state academy to dictate such things, however much some prescriptivists believe themselves to be linguistically privileged (note that even in countries with royal or state academies they, at best, manage to dictate just a part of the public sphere).

Insurance, underwriting, is only 'insure' in the US, however--that's an observation, not a diktat--so it's not as though the two spellings have the same range of meanings. Consider it lexical specialization superimposed on two spellings in what, at this distance, appears to be free variation.

It is the function of the US government to "insure domestic tranquility," but that has little to do with underwriting. Then again, the writers of the document that's quoted from were busy trying to abolish authoritarian rule in the US, so perhaps they missed the appropriate controlling authority.

Note that common American usage has 'cannot' written as one word, not two.

I can't speak to British usage in either case.