A leak could be coming from misalignment of one of the key pads with its hole or a problem with the pad material. If so, when you blow hard without depressing any keys, that note may sound.
I don’t think there would be any problem with the 2mm stripping, Andre. When I have replaced melodica gaskets, the new material is usually a bit thicker than what was originally there. And the stripping is usually easy to remove if you do it right away.
I am impressed with how thoroughly you have disassembled this Pianica. Did you keep track of the order of the keys, or were the keys of each type interchangeable? I always keep them in order, since on some melodicas there are problems if you don’t.