I have only recently used pastel but have picked up a couple of pointers. First, if using smooth paper, you can use a craft blade to scrape the pure pastel dust onto the paper, blending colours together with a tissue formed into a point. I have used hairspray which is cheaper than fixative but I did not know about the yellowing. The official line is to have one reverse mount with a second mount over to allow dust to fall between. Cover the drawing with clear perspex sheet taped at the back.
One other technique with pastel is to use a watercolour brush to wet the dust so that it becomes like watercolour. I have yet to try this in a serious work. Colourfix is something to try.
The blind leading the blind, but it may help.