On 1 and 2: It looks like we ought to revisit what should be done about non-standard metadata and, by extension, non-standard data arrays (with undefined NIFTI_INTENT) that might come up in surface files. We also add AFNI-specific meta-data into GIFTI files but we are not expecting them to be preserved through an exchange.

On 3: So are these normals defined once the surface is in xformspace or in dataspace? For the caret surfaces we have, where xform is identity, this does not matter. But in the general case, the meaning is ambiguous.

On 4: This is related to a previous discussion and is more of a stylistic issue. But to my mind, an empty value is like an uninitialized variable. So I opted to use 'Unknown' in my implementation and I gather this does not violate GIFTI edicts.