I just came across this interesting topic. I've had to learn a lot about building envelope strategies and moisture mitigation from designing and building our house in Alaska. I scanned this thread but probably missed some details - one of the biggest concerns with moisture/mold mitigation is your location.
Building Science and building envelope design are pretty deep topics, but in short, if it is cold in the wintertime, moisture in the air inside will travel through your insulation and contact your outer plywood sheathing. If it is very cold, and you have a lot of insulation, the moisture will tend to condensate on the cold plywood. If it is cold outside but you don't have a lot of insulation the heat from the room will heat the plywood itself enough to keep it from getting cold enough for moisture to condense on (avoiding mold, but costing more money to heat!).
One strategy is to use a 6mil plastic vapor barrier (VERY carefully detailed to avoid leakage) on the inside of the room under the sheet rock to keep moisture from being able to penetrate the wall cavity. This is effective, but of course acoustically it means that your fluffy insulation is not longer absorption for your room, because the plastic (and sheet rock on top of it) reflect sound. Some moisture inevitably still makes it into the wall, but it will migrate slowly to the outside through the plywood and water resistant barrier (i.e. Tyvek or tar paper) and diffuse into the air. Moisture in wall cavities tends to build up in the winter, but moves faster in the summer so in the end the wall doesn't stay wet long enough to cause mold or rot.
An alternative strategy is to forego the vapor barrier entirely. With this approach, moisture that condenses on the cold outer skin of the building will migrate to the warm, dry interior, where it is hopefully exhausted through an HVAC system or just from opening and closing doors and windows. This method requires a constant heat source inside in the winter (the reason we didn't go this route in our wood-heated house in Alaska).
Concrete floors are problematic because they can become moisture factories if they stay relatively cold, so that your warm interior air is always forming condensation on it like air does on a cold can of soda, etc. One can put foam and then wood on the floor to insulate it, or heat it well enough that the slab eventually warms up. This is why basements smell musty - condensation on the chronically colder concrete.
If you are going to try to avoid a hard interior skin entirely, you would need to be thinking about helping the moisture to dry to the inside. Mold resistant sheet rock is for bathrooms, greenhouses, etc., and isn't in itself a complete vapor barrier I believe.
In hot climates where air conditioning is always used this whole thing is basically reversed. The vapor barrier goes on the warm side, whether that's the inside or the outside.
It's really, really important to get this right for the longevity of the building and health of its occupants. Probably the best thing you can do is to buy some time from a local contractor to find out how this is handled in your area typically (an addage is that you want to find a contractor who has gray hair), or talk to an architect who designs for the local market.
Good luck. I'm looking to build a room about that size in the next year or two. It will be interesting to see how things go for you.