CLMM: Calculate ICC & Assessing Model Fit

The distribution across the 9 levels appears rather haphazard, which dissuaded me from trying a GLMM:

Maybe. A certain amount of that can be soaked up by different intervals between thresholds in the probit-normal - floor and ceiling effects on your scale for the most extreme categories is one example. The equivalent of this for an ordinary LMM is an inverse-normal transformation of the scores (normit/rankit). This sometimes seems a bit lazy to me, but as I commented earlier, you may end up with similar results to the more elaborate models. Worth doing in parallel, at least.
Cheers, David Duffy.