"too short"

Where? Not sure what you are trying to say.

I think all that matters is that it is square and lines up with whatever it is supposed to line up with.

Whatever it is - it is completely unnoticeable to anyone but you! :-)

Have the installer re-do it the right way.

That's hillbilly. Plumb it up and fix it right. Geesh.

The cabinet fronts aren't flush with one another. One is installed cock-eyed. The installer needs to fix this.

Since this is an island, the mess-up will be seen from two side, actually three sides because this is the end cabinet. Any linear design to the floor (tile, strip wood) will magnify the screw-up. And the toe kick will go wonky.

As Joseph said, don't hillbilly it.

Not much that I can do to fix the cabinet location. That was done last year and in between hospitals, the counter was put in and everything screwed down to the nth degree.

I’m the one installing the rest of the cover panels. I’ll take a pic later of what the filler would look like with the addl 3/4”.

The filler would be half that size but it gives the idea.

Well more gluing to make a filler.