I hadn’t thought about that issue with the moisture release mechanism. It’s worth also considering the Suzuki M-32C, which has a superior mechanism, the rocker arm that Suzuki has been using for many years. For it to work smoothly, though, the end piece in which it sits should be slightly rounded.
I’d like to see someone try encasing (in wood) a 25 key melodica. The best options would be the Yamaha P-25F Pianica or the Suzuki A-25C Melodion — same as Kawai 25-A but not in production since 1991, but I have an extra Kawai 25-A willing to donate itself to scientific research if someone is interested. The A-25-C/25-A is very similar to the current M-32C and is durable enough that it remains comparably functional.