"I am concerned about two things; because pine (for example) has a different
density than spruce, they might react differently to humidity or temp change
and affect the straightness of the spar."

I do not think the difference in density will be the problem. A thin piece
of dried wood, when attached to a larger piece of wood will continue to
adapt to the shape of the larger piece of wood. The larger spruce would not
be any more likely to warp - but for the fact that one side is now protected
from the moisture - and that would occur no matter what kind of wood you
use.

The completed piece should be no more likely to warp than the original
piece.

As to the second part of the concern - about the inspection - ask the
inspector in advance. Remember, it is experimental.

Colin