Scenario here being a situation where the Storm king wins and succeeds in conquering Equestria and the rest of the pony world (see "The Storm Kingdom" webcomic). Upon learning of the existence of the Mirror to the human world and being overcome by the idea of being an inter-dimensional conqueror. The Storm King would begin his plans to invade, subjugate and conquer humanity. Though, after learning that the mirror strips people of their magic when entering. He instead goes forward to design his own portal to the Human world. a much bigger portal big enough for his armies to march through and wouldn't strip him or his forces of magic when they undergo the transition of being humanized.

and so, how far and how much of the Human world would the storm king be able to conquer/destroy before both the human militarizes and the Humane 7 intervene? Not sure if nukes exist in the EqG world?

I mean just imagine the Whole of Canterlot City being converted into a giant Military Base/Concentration Camp where the storm king forces presses the enslaved inhabitants into his army.

There is also the possibility of Humanity and the Mane 7 being forced to enter Equestria in order to liberate it from the Storm King's reign. one thing's for sure, the Girls will not come out of this one unscathed and uninjured as much bodily injuries will ensure. Even the city of Canterlot isn't going to get out of this unscathed as it would definitely be devastated or destroyed in a show of power by said villains.

This sort of scenario would definitely convince Sunset to close the portals to Equestria and sever the link to the Human world as she would recognize the threat posed to Humanity by not only the magic of Equestria but also by the villains that inhabit it.

This also applies to a King Sombra, Grogar or even a Tirek Scenario. If these Villains opted to Build their own portals to the Human world (and consistently destroy/devastate Canterlot city int he process). They would be a threat to Mankind and would Justify Humanity's intentions of invading Equestria and exploiting it's resources as these villain's attack on Humanity would negatively affect Mankind's perception of the existence of Equestria. If Humanity survived these villain's conquests, due to the Humane 7's actions. What these villains did was awaken a sleeping giant and filled it with a terrible resolve. and that would also scare sunset, especially when she discovers how violent and bloodthirsty Humanity can be in comparison to the generally peaceful ponies. If our history of warfare says anything, we are not a "nice" race of beings.