No, maxsims, I’ve not done such an investigation, and I must admit that I don’t have the competence to do it. Having been Philippine-bound for most of my professional life, with only a few occasional opportunities to travel across the Pacific and across the China Sea as well as across the Sulu Sea, I really don’t know much about variations of verb usage across the oceans. Of course, when reading books and periodicals originating from the United States and from other English-speaking countries like Great Britain and Canada, I couldn’t help but notice the verb-inflection variations you describe in your posting. A tentative conclusion I made long ago was that these variations were simply part of the idiolects of each English-speaking country or region, and that they shouldn’t really be cause for losing sleep among grammarians and lay users of English. After all, practically all languages are like that—particular nouns, pronouns, verbs, adjectives, adverbs, or function words can vary from place to place.
In the six-town provincial district in the Philippines where I grew up, for instance, people in each town spoke a different idiolect of the Bicol language. They lived in an area hardly a 30 km stretch straight as the crow flies, so to speak, but they would pronounce Bicol and sometimes conjugate its verbs in profoundly different ways—and the differences grew in direct proportion to the distance. In fact, people in each town would often be wary and suspicious of someone who didn’t speak Bicol exactly like themselves—even if that person lived in a house just a kilometer or so from the border! Like a shibboleth, how you conjugated your verbs marked you. Your Bicol could be no less grammatically or structurally or semantically correct as theirs, but you could get ostracized or even mauled for speaking a different tongue.
It would be great, though, if a professional linguist--one who had looked deeply into your question before--would chance upon this exchange of ours and share what he or she had found about the whys and wherefores of verb-inflection differences in English usage.
