Sure he has it. He cut Monet's cheek to make her release Tashigi.
Well, you might be right. That doesn't really change anything, though. Zoro would just need to find the real body. He's been cutting nothing but animated stone so far. It's not like Pica could do much to Zoro, nor does he seem to possess any personality to push him out of his way to pursue enemies.
I'm not sure you quite need observation haki for that. It might be enough to have enough battle sense, especially now that they know it's happening all around them. As long as there's some tough guy nearby, anybody possessed could also be saved by severing the string. Also, I don't think Riku is worth anything compared to the real deals here. I guess he would be able to beat the noname pirates but that's it.