Hm, maybe the person Rose was facing was simply one of white diamond's commanders/quartzes? besides the triangle looks a little too big to be a gem? I mean its not impossible for a quartz to be the same color as her diamond, or have similar hairstyles.....
but then again maybe White's design simply wasn't fully finalized by then? could've been just a very devoted White Diamond fan girl commander, since the Diamonds themselves seem to so big and powerful that you can't face them in anything resembling normal combat.
but the other other hand....there seems to be a different part of the mural with rose in it depicting White Diamond in a similar pose to the four diamonds mural.....