You can ignore MUTF-8 and treat it as UTF-8. Spine's UI doesn't allow Unicode input anyway. Nothing in the binary format is specifically about Java.
Can't you use BinaryReader in C#? It looks like it is the C# equivalent of Java's DataInputStream, which is what spine-libgdx's SkeletonBinary uses.
I haven't written documentation for the binary format or SkeletonBinary for all the runtimes yet because the format is not yet stable (needs meshes, FFD, skinning) and also JSON is pretty ok most of the time.
A GameObject per attachment has advantages (eg attaching colliders), but I expect it will perform worse than spine-unity. Also it may be tricky to have attachments hidden and shown. As for it being in the official runtime, I'm not sure. I guess it depends on its complexity. No promises, but if it has all the features needed, is not a lot of code, and you would like to contribute it then I may take on the responsibility of maintaining it in the official runtime. If not we can still link if from the 3rd party section of the runtimes page.