I did, and i believe i completely understand it. The issue, as i see it, lies with fmc getting the usurped ability, meaning the slash must be linked to her and count as her "ability/attacking", as they even mention/name it in the chapter.
Let's put it this way: if you can acknowledge that the slash reservation skill is the result of the MC cutting through space and time, how do you explain that this somehow leads to the bestowal of a new skill/ability to Shizuka?
And why would it be limited to the amount of times the MC applied the skill to her? That wouldn't make sense if it's her ability.
As far as Shizuka getting the authorities go: it's not as straight forward as it seems. The
MC can immediately install an authority right after usurping it. Shizuka needs to
detune it first before it can be installed. On Shizuka's status screen,
the installed authorities show up as "pseudo authorities". While not as explicit as the other things I've said, I think it's fair to say that Shizuka is only allowed to use the authorities through the MC. I.e. when the slash hits an authority user, it goes to the MC, but Shizuka is able to "detune" it and install it as a pseudo authority.
So while there might still be some questions to the exact mechanics, it's clear that there's a difference between the MC usurping an authority vs Shizuka usurping an authority.
