I think it would be incredibly difficult to do in the modern world, and I think that all the above scenarios are vastly overestimating the influence of the few over the many.
In order to do it successfully, your best bet would be to undermine the current world social structure. That wouldn't be easy to do. The current world social structure is supported by literally billions of people - millions of whom have sufficient money and influence to hold significant sway.
For instance, while it would be incredibly easy to hold alot of power in North Korea merely by influencing the decisions of one or few people, North Korea is hardly capable of overthrowing the rest of the world's social order. Even if we move up to countries like Saudia Arabia, Iran, Russia, or China which do hold considerable sway, real power is invested in probably as many as 10,000 individuals and in order to control the country you'd have to hold some level of control over a significant portion of them. This would be easier to do if your agenda by and large integrated seemlessly with the agenda's already in place by the ruling class. In some cases, since the ruling class of several semi-autocratic countries does very much desire to overthrow the existing social order, this might not be too hard - but with the possible exception of China, none of the current semi-autocratic countries has enough clout to really put itself in a position for global dominance. In the meantime, you'd have to subtly manipulate thousands of ideologically motivated, ambitious, greedy, egotistical, and often backstabbing bureacrats separated by at times large distances. You can't cast 'charm person' over the phone.
By the time you get up to the USA, Japan, the UK and other G8 countries which wield real power, actual domestic power in those countries is vested in perhaps 100,000's of individuals and almost any policy which would would adopt would be opposed by a significant plurality of the general public. Any radical attempts to shift the policy of those countries - even in the face of apparantly compelling reasons - would result in massive outcry, protests, and if you weren't completely careful domestic unrest and ultimately populist revolution.
It's simply not easy to take over the world, and even having the ability to manipulate a handful of prominently placed individuals like puppets wouldn't make it any easier. There are limits to personal power in even the most autocratic states, and the world is as far from autocratic as perhaps it has ever been.
If you add to that a suspicious MIB like organization with knowledge of magic and the ability to work against it, I think that any attempt to take over the world via magic is pretty much foredoomed from the start.
If I were trying to do it, I would consider the necessary preconditions to be:
1) Total collapse of the economic system.
2) Total discrediting of free market systems, democracy, private land ownership, private gun ownership, free information flow and anything else that might act to distribute power and particularly decision making power to a wide group of individuals.
3) Introduction of a culture which favors command economic decisions, centralized power, collective land ownership, police power, gun confiscation, restricted access to the internet and other information distributing technologies (cell phones for example), and allows power to be centralized in a decision making few.
4) Hijack religious and academic insitutions in order to get them to praise the superiority of centralized decision making processes generally and you (or your puppets) specifically. This is actually one of the easier steps, because religious and academic institutions are predisposed to accept that decision making processes should lie in the hands of the few, and are especailly easy to sway with flattery and relatively small investments in money. Unfortunately, intellectuals tend to vastly overestimate the actual sway that they have over the general public and you could potentially completely subvert religious institutions and academia and find that the vast majority of the public was ignoring you. On the other hand, this step is still important because social pressure to conform is more important amongst aristocratic classes than amongst the lower classes and gives you the ability to leverage money toward your goals.
Once you get real decision making power concentrated down into the hundreds of individuals, its possible for a single individual to control society by controlling those individuals.
I don't want to give specific examples, because they would be political, but if you look at ways past dictators subverted populist or democratic movements you'll have a good idea of what is necessary. I'm not at all convinced that magic offers a significant advantage, except to boost charisma. Charisma boosting items are by far the most important magic items, because the most important skill you can have is public eloquence - particularly in oration. A person who isn't a gifted orator has zero chance of leveraging most modern societies significantly. All they can do is appeal to the expectations of existing subcultures - they can't create new ones on a sustainable basis. Charm person only works well in autocratic systems, and before any one person can claim real power over the system they first have to produce that autocratic system - by which time all the hard work has already been done.
I also question whether any such attempt is sustainable. I believe that it isn't coincidental that the more powerful the country, the greater percentage of its decision making power is distributed amongst a greater percentage of its population. In fact, I believe that the limits of human decision making ability being what they are, the more distributed you make the decision making, the greater efficiency the culture will have and the greater productively and creativity will be evidenced by the culture. So, its entirely possibly that your attempt to consolidate global power in your own hands is a self-defeating plan. The more of the world's decision making power is consolidated in yourself, the greater the advantage that the unconquered part of the world would have against your totalitarian centralized governance. Smart as you may be, you won't prove smarter than the distributed decision making ability of thousands or 10,000's of experts in narrow fields of knowledge. You'll make mistakes, and eventually you'll get overwhelmed by the cultural backlash against your increasingly obvious totalitarian ambitions. You might make a go of it, and you might wreck the global culture beyond its ability to recover for a very long time, but you won't end up ruling the world.
