The KH seems a bit on the high side, Mine is usualy 8-9 KH. You might want to consider slowly bringing that down. Are you adding pH buffers to the water? pH buffers are the devil (in the waterboy voice). KH is much more important than pH and when you add buffers it jumps up the KH. I originaly got a pH probe but after I realized that KH was much more important i've all but stopped using it. Haven't checked my pH in months. Also, any Nitrite is bad, but with such a low reading it could just be a reading error on the API kit. those colors aren't the easiest to read.
I've been having a similar issue with my zoas the last few weeks. I tried seemingly everything to get the to open, but at best a couple out of the 15 or so polyps would half open. What finaly seems to have gotten them to open fully is spot feeding them vs broadcast feeding. Might seem silly, but I swear I spot fed them with some oyste feast (as well as all of my other coral) instead of just putting the food in the water and within 2 or 3 days everything's polyps are way more extended. I've also been spot feeding every other day to every day. It's worth a try. Even if the polyps woun't open just squirt some food on them. some may get in, or they may smell it or however they find food, and open up. When I did it, only a couple polyps were even a little open, but the next day they were probably half open. then the next day all open.