So, many questions and so little time.

Nah!

Why do children ask why the sky is blue?

I'm not sure, but I don't think this should happen.  Vitamin C is very
perishable and oxidizes into a form (de-hydroxy-ascorbic acid?
dehydro-ascorbic acid?) which antagonizes Vitamin C.   In other words,
it's bad.  When it does that, it turns brownish, which is more
noticeable in solution than in the dry form.

Can you try dissolving each of them separately in water, to see whether
it's one or the other?

I've kept niacin (not the amide) without refrigeration for long
stretches of time, and haven't noticed any change in the way it causes
the niacin flush, which indicates to me that it's still good.  But I
don't have any experience with niacinamide.

(In general, when white things change color, they're no good any more,
but apparently there are exceptions, such as PABA.)