It’s very common for unis to be assembled backwards. Bike shops often don’t know that it’s possible and problematic. (It doesn’t come up when assembling bikes).
The location of the frame slot doesn’t really matter, but it’s best to keep it to the rear so it’s clear what the rear is supposed to be. The thing that really matters is your pedals; if they’re on the wrong side, they’ll unscrew and ruin your cranks.
If I were you, I’d flip the wheel around so that the right pedal is on the right side when the seatpost slot is in back.
The slot at the front or rear should not be an issue. It is probably supposed to be at the rear but its purpose is to allow the frame to be clamped down onto the seatpost and thus direction doesn’t really matter.
What is critical is that you keep the cranks on the correct side. The pedal threads are directional and can unscrew if backwards. A self unscrewing pedal can destroy the cranks.
I took my Coker in to the LBS to have a broken spoke replaced (I had to provide the spoke of course, they don’t carry anything near that length), and they put the wheel in backwards afterwards. I wouldn’t have noticed for a while, except I have to remove a pedal to fit it into my (small) trunk, so I had them switch it back right then. So yeah, I guess it happens a lot.