dry cellulose or wet?
did you say 2x2 of cellulose? that will get you about R6 or 7 in addition to your clay wall?

i think you would want to understand your climate to make the choice. both materials can be hygroscopic. which will take on and give up moisture at a quicker rate? should this go to the oustide or inside based on climate / interior conditions / etc.? my guess would be cellulose inside, clay out.

and - why would 12" think cellulose pose moisture issues? you'd want to use sticky cellulose to keep things from settling and to provide a tight air barrier...

here's a friend who is doing a 12" thick wall with conventional construction - 2 walls framed with a spacer:
blog link