Should a retouching varnish always be removed before using a final varnish? Or is it okay to varnish on top of the retouching varnish once the painting has been allowed to cure?

It is designed to be varnished over. It can be used as temporary protection if you have to show a painting before it is fully cured. Then when you are ready for a final varnish, carefully give the painting a quick wipe with a solvent to remove any built-up dust and do a final varnish. Some artists also use it during their painting process to oil-out sunk-in areas, but many brands specifically mention that this isn't its actual purpose. But it does add gloss to matt areas.