Harmon Should have taken Josh up on his offer......

to meet in the middle of nowhere. Harmon is in a really bad spot. If the police catch Josh, he's almost sure to roll on Harmon about the dam to get a reduced sentence. If the police don't pursue Josh, it will be because they focusing exclusively on Harmon because of the outgoing calls from Dena to him and even though he's innocent of the murder, he can't afford that kind of scrutiny lest they uncover his involvement in the dam.

Harmon's best move would be to spin a yarn to the police about Dena confessing that she and Josh blew up the bridge and that she was losing it because of her guilt over the hiker. Say she told him that she tried talking to Josh about it but he became cold and threatened her. Say she told him she's scared of Josh. That way the police are focused entirely on Josh. But of course they can't actually find him or Harmon is screwed. So he should have agreed to the meeting, killed Josh, and buried the body somewhere it will never be found. Police assume Josh murdered Dena and then went on the run and they spend the rest of their lives searching for him.