Your concerns are entirely justified. Your "contractor" has already demonstrated by his actions and his "product" that he lacks the knowledge and competence
to be installing tiled shower receptors. Industry standards for such installations are to be found in the Tile Council Of North America Handbooks, for instance, and no doubt the
building and plumbing codes in effect in your area have very similar or identical requirements for site-built showers. I take it there was no inspection or leak test
performed on the shower pan? Those proposals by your "contractor" sound like hare-brained acts of desperation to me. I would demand that he provide proof that any
substance or method he suggests is warranteed by the manufacturer and approved by a recognized agency (such as the UPC) for the intended use. Or you could just
find a different, qualified tile professional to remedy the situation, although that will cost you at least in the short run.