As an example of Orwell's annoying quality -- how much does Chesterton really have in common with Communism? Maybe our reaction is influenced by the fact that we don't take Chesterton wholly at his word (and maybe Orwell does mean Belloc, rather than Chesterton), but still it does look like an outrageous comparison from where we sit today.
Also, Orwell's distinction between patriotism and nationalism captures something real, but also distorts or at least shapes reality to suit Orwell. By definition, in his view, the nationalist seeks power and wishes to impose his way on others. By definition, if you are not obsessive, unstable, and indifferent to reality you are not a nationalist. But this imposition of one's own categories can do violence to reality. What to make of Madison, Hamilton, Marshall, or Monroe, who were all nationalists to varying degrees and in ways, but who didn't have all these characteristics? One could say they were patriots, but that doesn't quite get at one side of their active life, which "nationalist" does. Perhaps what Orwell is talking about is frustrated nationalism, which feels its wounds intensely, rather than a healthier form, which doesn't focus on wounds and victimization and doesn't have to prove itself.
I think he takes that criticism into account somewhat when he said that nationalism doesn't exist in a pure form, but in degrees(although sometimes in very large degrees) and often intermitantly. If I can answer you on the Founding Fathers the way Orwell might have, it operated on them only at times, and in relatively small amounts, as a tendency within their overall patriotism. The effect was to push them against compromise at key moments.