They are going to find her

I hope that you are right (truly) but, I fear that may not be the case. The film made it seem like the entire town would go there (randomly) to drop off the crap they didn't want into the massive sinkhole whenever they wanted to. It did not seem like anyone was supervising what was tossed in or, "who" was doing the tossing. I assumed (for some reason) that eventually the sinkhole would reach the "crap limit" and would then be filled in with dirt.

This is just my opinion from watching the film, I haven't read the book. The scene where the killer throws the body trunk into the sinkhole, showed that it continued to descend further towards the bottom. I felt like the director was telling the viewer that she would never be found.

I dont think so. Not right away. I think what''ll happen is he will realize what happened, but keep it to himself. At some point in the future he may mention it to his daughter and she'll be the one to get it dug up or just place a grave marker. But that sink hole was so deep and the vault sank into what looked like quick sand that it'll probably be too unsafe or too time consuming for the authorities to bother.

But he's not wanted for questioning. He lives a full life until he stumbles off a cliff, decades later.