I would prep the seat pan. Sand off all the old adhesive and paint. Use rough paper to allow the adhesive to get into the metal and stick. It's all about the prep work.
Heat molded seat assemblies can be a real pain to work with.
For extreme cases, I usually rough up the plastic or sand the metal and then use expanding foam in a can with a putty knife. After that I use plenty of gorilla tape to hold it together until the foam cures. Once cured, the tape is removed.
It takes a little practice to realize how the expanding foam works and holds. That stuff does not give up once is sticks the seat materials together.
