Normalization and UTF-16 are completely orthogonal. It doesn't matter in the slightest whether the filesystem stores the filename in UTF-16 or UTF-8, either way it's still a sequence of Unicode scalar values, and that's what matters.

HFS does a lot more than convert to UTF-8. E.g. it will normalize ö to o+(umlaut), using custom normalization rules that nobody else uses.

If you tell HFS to store a file with an Ö in it, and then list the directory, the character Ö is nowhere to be found.

Is it stored "real UTF-16" or in USC2 wide-chars like in NTFS? The later would be a big problem.

I believe HFS+ was created before Mac OS supported surrogates.