Why do grammars claim that adjective+adjective is always a morphological compound and never a syntactic construction?

Solution 1:

The biggest clue is that the terms white-hot etc are hyphenated.

Certainly, something could be both white and hot, and so possibly off-white and mildly hot. In those cases, both parts modify the noun phrase that follows.

Consider one of your examples:

very light and very dark blue crayons

If this was simply a transcription of something spoken, one could conceivably parse the original as any of the following:

• (very light) and (very dark blue) crayons;
• very light- and very dark-blue crayons;
• ((very light and very dark) blue) crayons; or
• (very light and very dark) (blue crayons); etc.

None of that says anything about whether dark-blue is composite or compound.

However, if you were given the text dark-blue crayon, you don’t have a crayon that is dark and that happens to be blue as well. The crayon is modified by a single adjective, dark-blue.

Note that colours aren’t a good example for the 4-way non-syntactic test you quoted because colours often have modifiers that produce other colours that happen to fit the pattern without making the result non-syntactic. For example, blue and navy-blue are both colours, so one can have dark-blue and dark-navy-blue crayons without needing to assert that dark-blue has passed one of the non-syntactic tests.

To answer the question in your post’s title with reference to the examples given early in the text of your post: hyphenated words in English are always treated as compound words, never as separate modifiers.