What about when there's a country, say... Iran who doesn't want to join, and neither does their population. Will the mega-country just walk in and assume control?

Not MY vision of the mega-country. I believe that, if done properly, a one-world government would have significant economic, logistic, conflict-resolution and technological advantages. These would likely be maximized by getting EVERYONE in on it, but something as large as you are talking about should already be in a better position than any of the original nations were. And I suspect, over time, the citizens of Iran would come to desire these advantages and start moving to the other country or petitioning their government to join. No hostile takeover should be necessary or desired.

And to your first point, you said 'When you strip away all political intricacies and ethnic hatred'. You can't do that!

The political landscape would change. It would be harder to find enemies by scapegoating other nations. Chances are political parties would kind of emerge as the new bogey-man for politicians, but it is already that way anyhow, all over the world. Personally, I would hope for the scenario to foster direct competition between politicians vying for an open position, and that the competition would become much more heated and grand as they work past the local regional ("national") level. This should encourage them to work harder for their constituencies and the world simultaneously, and in turn benefit society as a whole. Maybe not though. I never claimed this whole thing is not an idealistic notion.

As far as ethnic hatred: it will be a sad day when I admit that there is no way humanity can finally be done with that ancient affliction. I have hope, perhaps too much...

You did an outstanding job on finding conflicts that were not (at least ostensibly) about resources or land (although I'm pretty sure conquering the Holy Cities and key production facilities was actually the true reason for the Crusades).

But behind several of these you find causes that, while perhaps not exactly about resources, were still political power plays that can only be fueled by warring nations without a higher government providing checks and balances. World War I and the Cold War immediately jump out. And civil wars...well those so often tend to be de facto wars between classes. This, I admit, would not likely disappear in my vision of the world government. No government can solve all problems, and I never claimed conflict would disappear. I just believe that it can be much reduced by my proposed government.

And yet there would still be some who would refuse, those that see the oil as theirs to monopolize, those that would not play ball even if it was in their best interests.

I'm not sure why you say this. Sitting on a surplus of oil would cause you to bring in less money than you could be making. Also, in a directly linked economy, the weak links would pull the whole down a little bit. There would be pressure to play ball, and not just governmental pressure, but pressure from the local citizens and workers who might start thinking about heading somewhere where resources are being used more productively.

So really then, this isn't about having one country, this is about having one economy. This can be done without the abolition of all countries.

That is a huge part of it, but by no means all of it. No borders means no need for temporary visas or to gain citizenship anywhere. Restricted travel zones would be limited to areas of natural disaster rather than decades old failures in diplomacy. Information networks, scientific endeavors, shipping agencies, all would have less red tape to cut through, more options for expansion and cooperation. And I still argue that conflict, while not eliminated, would be limited.

Besides, a truly global economy unfettered by politics while hundreds of nations exist seems quite unlikely. With individual countries participating in the economy you will still have trade restrictions, many of which are based on old disputes or attempts to strongarm certain governments that don't do what the powerful nations want them to.

While there is of course indoctrination in the world, implementing the idea of universal equality would be the single largest indoctrination in history.

I am not proposing some sense of universal equality per se. There would still be plenty of variations among the population. I am simply stating that these nationalistic precepts we cling to do more harm than good anymore. Erasing borders may make us "more equal", but does not require indoctrination. It simply means erasing some of the current indoctrination already happening and increasing our overall freedom.

You would have to brainwash people out of natural instinct

Our so-called instincts are much more mutable than those of any other animal. We have reason, creativity. We have history books and computers. We can see other options, more clearly now than at any point previously. The reason these "instincts" are present is because they have historically served us well. Resources are always limited and distributed randomly about the globe. In the past we always had to hold, protect and capitalize on whatever resources were to be found in the area that we settled in. And most wars are fought over that. But now, resources can be sent anywhere. People can go anywhere. Information can be had by all. The game has changed so dramatically that the old rules can be (and almost certainly need to be) rewritten.

But then you'd have to make everyone agree on what economic system to use. Some would say capitalist, others would say communist. Some would say keynesian, some would say Austrian.

There would room for experimentation zones. China already does that and it has helped them immensely. The problem there is you pretty much have to participate by the rules of whatever zone you live in. People who didn't want to be in an experimentation zone wouldn't have to be, as there would likely be replacements for them from all over the globe. Also, as the transition process occurs, many political aspects that limit economic activity would disappear and you could start looking purely at the data instead of having to factor in treaties, tariffs and whatnot. Whatever would be most effective would eventually win out. For all we know, it would a whole new economic school of thought that had never been proposed yet because it was never possible under the old system.

My problem is actually getting universal agreement on everything.

We wouldn't need universal agreement on everything. Just a basic set of rules and procedures designed for facilitating global transfer and ensuring aid in case of emergency. Maybe a central information network, a centralized transport authority, but a lot of things can be left up to regional authorities in much the same way that states in the US are allowed to pick their own tactic for many issues.

All reasons for conflict among humans are petty, yet we still don't see it.

As I stated before, they weren't ALL petty. There were legitimate reasons for nations and war and all that jazz throughout human history. But now those reasons are disappearing as we transition into the information age. Part of the reason we never saw the pettiness is because at their core, the old ways really did help us get by in a larger, more distant world. Now, I believe, they just hold us back.

To solve disputes over land

Those should not exist on a national level to begin with, at least not anymore. Removing national borders would move land disputes down to individuals and can be solved through the legal system. And if someone lost the land they wanted, there are billions more acres out there they can try for instead.

to test new economic theories

I already basically responded to this above.

to give people a sense of pride (which I see as foolish, but people sure as hell love stuck up patriotism).

So you see this as foolish, yet you you trot it out as a reason to support your stance? Isn't that a bit like intentionally hiring incompetent workers to build your house?

By eliminating these things, you make everybody more similar. Whether these things encourage group conformity or not is irrelevant, the fact is that they give humanity some diversity.

More similar does not mean entirely similar. Are you honestly proposing that the only differences between us are the largely illusory ones propagated by unnecessary institutions? Further, co-mingling of different systems increases diversity in the same way that combining blue and yellow create a whole new color. Except in this case, the new colors in our palate would be almost limitless.