Nintendo is a monopoly with respect to Nintendo games (but not Nintendo devices, since you can buy those all over).
Monopolies aren't inherently illegal. See e.g., patents and copyrights, which are legally granted or protected monopolies.
What is illegal is anti-competitive practices. This doesn't require a dominant or even majority market position--authorities in the US and EU have gone after companies with relatively small market shares that engaged in anti competitive actions (though they haven't done so in many years).
Yes it is. Nintendo is a monopoly with respect to licensing games for sale on Nintendo consoles. Monopoly is market-specific under US law, and the regulators can get very specific.
Regarding the first element, it is "settled law" that the offense of monopolization requires "the possession of monopoly power in the relevant market."(5) As discussed in chapter 2, monopoly power means substantial market power that is durable rather than fleeting--market power being the ability to raise prices profitability above those that would be charged in a competitive market.(6)
Monopolies aren't inherently illegal. See e.g., patents and copyrights, which are legally granted or protected monopolies.
What is illegal is anti-competitive practices. This doesn't require a dominant or even majority market position--authorities in the US and EU have gone after companies with relatively small market shares that engaged in anti competitive actions (though they haven't done so in many years).