I am American and American English is my mother tongue. Growing up this way, I feel that Midwest American English sounds very neutral and middle-on-the-spectrum. What would you consider to be the most neutral version of English (if one exists)?

I'm not a linguist, but am pretty sure that everyone's brand/dialect of English sounds the most neutral to them. There is no normal, in other words. As to your second comment about separate UK and American Englishes, I'm reminded my father's experience on a training course he attended for new employees of a large insurance company. He, a recent immigrant from Belfast, Northern Ireland, was paired with another new hire from Birmingham, Alabama. They got along just fine, but had to resort to writing notes to each other on occasion…!

I am English and English is my mother tongue. Until I began to think logically about the situation, I felt that Recieved Pronunciation was the most "neutral and middle-on-the-spectrum". When I was growing up, the only American accents I heard were oon TV so subconsciously I associate American accents with acting. One of my best friends is from California and even though I've known her for 10 years there's still a feeling in the back of my mind that she's putting her accent on. In order to imitate her, I have to strain my natural voice to copy her "weird" o sound. She says "bahx" instead of "box", "budderr" instead of "butta". But she has to do the same to imitate my "weird" accent. There is no such thing as a "neutral accent". Everyone has one , no matter how neutral they think their accent is. Familiarity with your own accent blinds you to the fact that it is an accent.

Spanish and Portuguese were born as different languages, though they both came from Latin, like French and Italian. Spanish was closer to the central part of Europe, so it was also influenced by Celtic languages, whereas Portuguese was more isolated and developed accordingly. They sound very different and have distinct grammars.

Bill Labov is the expert on this topic. He does identified the most neutral accent in the US. This is what broadcasters often shoot for. But the standard changes through history, and major changes are underway. Maybe he doesn't identify the current location in this video. But in some video he does narrow it down.

I'd agree with what you say about the Midwest accent. I'm from the UK but I've spent a lot of time in the US since my ex-boyfriend is from Ohio. He studied in California and his sister studied in D.C. and both of them have received comments from their friends about their accent - mainly to the tune of 'You don't have an accent at all'. Of course, this is ridiculous, because from my perspective they do have an accent, everyone has an accent (they even exist in sign language) - but within the spectrum of American English, I'd say yeah, the Midwest is probably the most neutral, 'accent-free' zone (although you also have to bear in mind that middle-class accents tend to be more neutral than working-class accents, which often have quite a strong twang, certainly in Ohio!). As for US and UK English eventually diverging, I agree with Raj Bhuptani below - as long as the nations remain politically close, the languages probably won't diverge. The separation of a language into two is often much more political than linguistic - for example, Prof Ridjanovic from the University of Sarajevo says in his book Bosnian for Foreigners that the differences between the languages of the former Yugoslavia are actually fewer than the differences between the Englishes of the UK and US, and yet Bosnian, Croatian and Serbian are treated as separate languages while UK and American English are not. In many cases, it has much more to do with the political history of the countries involved than the linguistic similarities or differences between the languages, so it's fair to suggest that as long as the countries remain close, so will their Englishes. I'd also point to the use of English online as another factor that would probably inhibit the separation of the two languages any further - people use their native dialects when chatting or posting to Facebook, etc., but most 'formal' Internet content is written in standardised UK or US English, and the differences between the two are unlikely to widen while people are constantly sharing and exchanging so much content online. If you look at language evolution in the same way as the evolution of different species, in most cases one species splits into two partly as a result of some kind of isolation of one half of the group from the other. I expect language evolution follows similar lines, and US and UK English are less isolated from each other than ever.