What type of glue are you using to bond the shark to the cow hide? Try barge cement also try cleaning it with alcohol before you glue it. When well bonded no amount of tight stitching should pull it from the edge. As for burnishing, I haven't used any shark, but with other exotics something that has worked for me was burnishing, then sanding with fine sand paper like 400 grit or something then repeat until you have an edge you are happy with.
Looks good by the way, that caiman taser holster is SICK! What weight would you say the caiman skin is?