I feel your pain, I'm amazed someone would just use tiger seal and then leave it at that, surely it should have been filled and then painted over?! And what moron doesn't dry fit first to make sure it doesn't impede the wiper.
I had the same spoiler fitted to mine a few weeks ago, but the poverty spec version not the carbon fibre. They had to take off the standard mini spoiler and prep the area first due to the standard running the full length of the boot, whereas the new one didn't. With mine being black it hides any imperfections but like you I have gone over it to see examine the workmanship.
Rightly so getting your dosh back, but it sounds like you were a bit unfortunate with him getting the 'hired help' to fix it.
A decent body shop will get that off, prep the area and repair any damage, mark it up dry, fit it properly and then blend it in. Not sure if you have found someone already to do it but I always like to find someone with outstanding google reviews, fortunately the guy that did mine was more of a perfectionist than me.
Good luck and hope you get it sorted.