1) In the 10th century, (Old) English and (Old) German were already somewhat divergent, and not really mutually intelligible, but closely related.
2) The same goes for Norman and French. While in the same language family, these were not quite the same language.
3) Have you read any Chaucer? I'm not sure I'd call Middle English "mostly comprehensible". It takes a lot of work and a good ME dictionary at hand. By contrast, Early Modern English (think Shakespeare) is more in the line of "mostly comprehensible".
4) It's not just the fact there was no central authority that caused English to massively change. There still isn't a central authority, but change has been slow and non-radical for the last 500 years or so. Having a central authority doesn't really prevent change, either, although it might help to slow it down.
5) It's not at all clear what you mean by "arbitrary" in your claim about language change and state control, but you certainly have not backed up any claim about languages under "state control" being more "regular".
6) The alphabet switch for Turkish did not succeed because the Latin alphabet is "simple and phonetic". The Arabic alphabet is also simple and phonetic, as is (or as could be) any alphabet. It was not a great fit for the Turkish language, however, and the Latin alphabet (with a few additions) mapped better.
7) But much more important than any facts about the Latin alphabet was the fact that Kemal Ataturk was a dictator who pushed through a massive literacy programme on the population (who, it should also be said, was basically supportive of this goal). Had he done an alphabet reform instead---adding a few letters to the Arabic alphabet to make it a better fit for Turkish---and accompanied it with the same literacy policy, it'd do just as well. (Better, arguably, because it would have left the writings of the Ottomans much more accessible to the literate modern population.)
You might be right about computers arising in alphabet countries (I'd broaden that to any non-ideographic writing system, including Greek, Korean, and those of the Indian subcontinent and southeast Asia), but it might be more accurate to say that if computers had arisen first in China they would have looked very, very different. (Even in Japan, they might have gone the route of kana-only systems first, in a similar way to how early western machines had ALL CAPS interfaces.)