Three things about fixative:
1) Some do work well for use while working between layers.
2) Most seem to work poorly for a final protection layer.
3) Spectrafix Natural Glass Final Fixative is the one I found that doesn't smell (it's milk protein based), is non-toxic, and if used carefully and properly does seem to work pretty well. I use it carefully misted at a distance with 3 or more very light coats and that works very well almost always.
But I have found none that is always perfect, nor do most reviewers.
As to glazing, do NOT use plexi nor any form of plastic EXCEPT Optimum, which is designed to resist the static that normally plagues plastics. That static is very bad for use with dry pastels. Glass works best, IME, and frankly, Museum Glass or other high quality "white glass" with non-reflective coatings. Those do show off the brilliance of pastels properly, but unfortunately they are expensive.