I totally hear your concerns on the MKIII. I wouldn't worry about the construction at all, the factory has been producing the Nacra 15 and Nacra 17 for over a year now and the quality is way better than the previous boats. I've had numerous people mention how good these new boats are.
The new open daggerboard case is a very cool design to allow the boat to take curved or foil boards down the road. However as they found at the Europeans, water was shooting up them and slowing the boat down. As such, Nacra has come up with a simple solution that is an insert attached to the bearings. This will make it impossible for water to come in.
Removing them will be simple, removing 4 screws on top and bottom then putting in the new bearings for the board you want to use. The bearing would have to be changed anyways. Honestly, I think this is kind of cool because it gives you an extra sacrificial daggerboard case. The first boats all weighed in an average of 2 KG underweight so adding this will not be an issue.
Feel free to give me a call if you have any questions, I can send you my number if you email me at firstname.lastname@example.org