First let me point you to this thread where all of these issues have been discussed already.
But as for your options:
A and C are highly unlikely.
Why would Mexico be part of the same government as the US and Canada? If Spanish wizards never arrived in the New World, then they would probably have Aztecs and various other people running whatever governments are in existence.
If the magical US managed to maintain the same borders as the Muggle US, then they would have had to exactly duplicate the wars against Mexico, to gain the exact amount of territory as the Muggle US, including the Gadsen Purchase. I see no reason that wizards would ever buy the same land as the Muggles.
This isn't even addressing Hawai'i and Alaska or the various cessions that the Muggle US has gotten through treaties with Britain.
B is the most plausible though there's no reasons that these borders have to have any rhyme or reason when compared to Muggle borders or Muggle history... e.g. Polish Florida!
I'm not sure where you're going with this term oligarchy. People can claim that the modern Muggle US is an oligarchy because to get anything actually done you have to be fabulously wealthy.
It is never, at least to my knowledge, discussed how exactly the Minister of Magic in Great Britain is chosen. They might not have a democracy either, at least not in the sense of a popular vote. Check here and here for more information on that side of it.
The shape of a particular government is intimately related to the history of an area. What do you think wizarding American history is like? Were there British settlers or did the American Indians fight them off? Was there a treaty signed? Did the British obey the treaty? Were there French or Spanish or anyone else trying to settle there? Would Spain or Mexico have retained greater lands in the western part of the US?
Why were the British wizards leaving Britain?
Early British settlers in the US try to get rich (gold, cotton, tobacco, etc) or were searching for their religious freedom (Pilgrims).
By the time the Founding Fathers roll around, they've absorbed a lot of Enlightenment talk about democracy and power for the people.
This doesn't mean wizards would have done the same.
Would all of these areas have different histories that would make them less inclined to work together? e.g. multiple countries Or would they try to work together in some form of federation of states? e.g. more like the structure of the Soviet Union.
I highly doubt that a strong federal system like the government of the modern US is what wizards would have.