Yeah, I agree he could end up at 12, but for now, he's an OC playing out of position - even if only in my mind when I see him wearing the 12 shirt.
This season he's had 4 starts at 12, 6 starts at 13. In total he's had 26 starts at 12.
Slade on the other hand hasn't started at IC for a while, but has 20 starts at 12 (about half of them at international level, which counts for more when talking about international selection). I wouldn't say that "Dingwall has played 12 way more than Slade"
Either way, both would be out of position (though my suggestion is to play him alongside Lawrence, with the specific aim on not nailing either of them into the IC & OC role).
ETA: stats from all.rugby btw
I also acknowledge my bias that I've said for... about a decade now, that Slade has everything I want from an IC - except experience at IC.