It might be a question of definitions to some extent - a dialect for me is primarily about spoken language, as even languages with only a single written standard can have tons of different dialects. Which includes British English - there are plenty of different British dialects even within the UK. I don't know too much about American dialects, especially not Southern ones, so I couldn't possibly say how valid it is to view Southern as a single dialect which is hence the biggest in the USA. But I do agree that if it's the biggest in the USA, it's probably also the biggest in the world. At least before you go into the question of the non-native speakers.

You'll note that I'm not talking here about continental Europeans - that's because things are different there. The large majority of English speakers in India, Kenya, Nigeria have other native languages, just like the ones in Europe, but the difference is that those three countries (and many other African ones) do use English as a lingua franca - much of the media, books, workplace communication, and so on is in English. Which means that e.g. Kenyan English has standards and norms of its own, and someone who speaks a different kind of English will stand out. For e.g. "German English", this is not the case - there is a German accent, sure, and perhaps even some words which German speakers are more or less likely to use - but those are markers of imperfect language skills rather than markers of a separate variety of English: as a German improves his English, those things will fade away.

And then which English will he speak as his fluency improves to near-native levels? Depends. Can be American as well as British English, or more likely something inbetween that is neither one nor the other. The most decisive factor would probably be who he spends most of his time talking to - Americans, Brits or other European non-native speakers.

It all depends on how you define "fluency", but it's worth pointing out that most numbers I've seen estimate the amount of "English speakers" both native and non-native at no more than about 700 - 800 million globally. Which makes clear that certainly among the Indians, only a minority is counted.

More like 50% based on the above.

Well, you have a point for cases like Kenya that I discussed above. But for countries where English isn't used in daily communication, I'm less sure. Even if you do count the fluent English speakers in those countries (and where does one draw the line of "fluent"?), it would be quite difficult to figure out which dialect they all speak.

To answer your question in the second post - in the English I was taught in school, the aim was to make students pick either British or American English and stick with it. Many Europeans, myself included, have a surprisingly hard time distinguishing between the two, because we constantly hear/read both, and both are familiar to us. So nearly everyone ends up with something inbetween, both in accent and in vocabulary, unless their individual circumstances are such that one of the two predominates (e.g. having British family, studying abroad in the US,...). In my personal case, the default is somewhere halfway, but when speaking to Brits it shifts in the direction of British, without ever quite getting there, and the same with Americans.

Any question of dialect size will depend on ultimately arbitrary decisions about where you draw the dividing line between one dialect and another - the infamous "dialect continuum". I think it doesn't make any sense to claim that all of the people you listed speak the same dialect. But, as mentioned, I don't know whether it does make sense to claim that "Southern" is in fact a single dialect.

The last point, though, I completely disagree with - dialect certainly varies more within the UK than within the US. And within the US, it's not a coincidence that New England has more variety than the West. It's all a question of history - having more time to develop dialectical variations, and the amount of geographical mobility, which if high will rapidly erase dialect differences between regions. In Europe and to a much lesser extent in New England, you may find places with such low geographical mobility that to this day the local dialect is easily distinguishable from the one spoken a mere 3 miles further, with entire dialect groups living 30 or 50 miles apart. That's absolutely not the case in most of the US.