1 and 2. Those items you list are from Caret's long existing metadata.
Each of Caret's data files has a "header" that contains name/value
pairs much like GIFTI Metadata. When Caret writes one of its data
types as a GIFTI file, it places its "header" data into the GIFTI
Metadata. As stated in 2.10.1 of the GIFTI Surface Data Format
document, "Metadata elements with names not recognized by the
application software should be preserved or 'passed thru' for use by
other neuroimaging software."

3. Prior to "Intent Codes" GIFTI data arrays had an attribute named
"Category" and one of its values was something like "Normals". When
the decision was made to switch to "Intent Codes", we (I believe Ziad,
Rick, and myself) chose to use NIFTI_INTENT_VECTOR for normal vectors.
As stated in 2.3.4.9 of the GIFTI Surface Data Format document,
NIFTI_INTENT_VECTOR is described as "data consists of three-
dimensional normal vectors". An intent code such as
NIFTI_INTENT_NORMAL_VECTOR would be better, but it does not exist.

4. Would a metadata value element that is empty imply that that the
value is unknown or unspecified?
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John Harwell