Confused with Fighter class features

[sfedi]

Basically, if an opponent that the fighter has marked does a shift and the fighter hits with his interrupt attack, that particular attempt to shift is canceled, ...

No he doesn´t.

That happens only on an OA, which this is not.

This is a "melee basic attack against that enemy as an immediate interrupt".

 

[Mirtek]

Basically, if an opponent that the fighter has marked does a shift and the fighter hits with his interrupt attack, that particular attempt to shift is canceled,

No, the stop movement part applies only to attacks combat superiority. The attack against a shifting foe from combat challenge does not stop the shift. Unless you kill the enemy with this attack, he will complete his shift after the hit