There are merits and demerits to the points you make.

While it is important never to exclude the events of 7th of December 1941 from history, America had begun to make plans to support Britain since the late 30s.

Yes, you are right to point out the great work done by Alan Turing and the code breakers at Bletchley Park, it would have amounted to nought, Had America not supported Britain and America.

But the relationship between Churchill and FDR was fraught with tension on the question of the empire.

And WSC wasn't happy about that.