Mounting a 1-gang alarm panel over a 2-gang hole
Well, it seems unlikely to be easily solved with a 2-1 gang reducer, or at least 
I've not met one that also turns the single gang 90 degrees. With a 4 or 4-11/16 square box you just use a one-gang and it could be mounted either way, but that's not what you have.

Three options:


Hot-glue (or epoxy) the nuts to the back of the faceplate.
Place the alarm panel slightly above or below center so that you can
drill holes in the alarm back to access one set of mounting screws
through it, and access the other set directly above or below the
alarm panel.
If the LV ring is square, (they are pretty close to square, but sometimes just a bit off) unscrew, remove, rotate, screw in, and then use a 2-gang to 1 gang "centering" trim plate, which are commonly available.