I would definitely agree with you that now it would be a daunting task to try and find her in hiking/mountain areas. You not only have weather issues but wildlife issues as well as far as things that could've helped in hiding or scattering a body.
As far as Fred goes, I think it was more of a stategical move on his part early on when his daughter went missing to divert the attention away from the happenings that were going on in Maura's life prior to her disappearance. He wanted the invesitgators to treat the disappearance as a crime as opposed to a voluntary run away case. He knew that if they viewed it as a crime, that they would be more involved in acutally trying to solve the case and would provide a healthy amount of resources and attention to it.
If they thought she left on her own accord, then they wouldn't volunteer nearly as many resources or the time to solve the case.
If Fred kept pushing the boogey man theory, he knew the pressure would stay on the police and investigators that they were dropping the ball.
It's been interesting to read Mr. Renner's blog and read between the lines a little bit about the family keeping quiet for one reason or another, but up until recently I've never questioned it and have always thought it was a case of just what you said: trying to get the police to take it seriously.
My husband went missing in a dangerous area of our city. When I filed the missing persons report, they, of course, asked me if we'd been in an argument. We'd had a little argument, yes, but I knew it had nothing to do with whatever happened to him. I was reluctant to bring it up when they asked, because I knew what they were getting at. From the beginning, whether I would admit to an argument or not, they wanted to believe it was a case of the husband wanting some time away from his wife with whom he was probably irritated.
Had I explained every detail of our argument and had he been missing for any longer than he was, people would begin looking into all kinds of irrelevant things that really had nothing to do with what happened to him.
Another thing going through my mind at the time was that even if they had been right about someone initially going missing due to an argument, I knew that in our area in the middle of the night, without a working phone, it was unsafe for anyone to be out, regardless of the reason. This is similar to Maura's case in that LE very well could be right that some things were upsetting her (well, clearly they've got that right since she left school), but her family may believe, like lots of us do, that whatever happened to her happened after the wreck and the reason why she was where she was is irrelevant.
The family might have feared/might still fear that if all details of what was upsetting Maura were revealed, LE would automatically dismiss the case as a suicide, even though something else could have happened to her that was unrelated to her being upset.
It wasn't a case of me holding anything back, because yes, you want to do everything you can to find your family member. In fact, that's exactly why some people don't want to go into details that they don't believe are important, whether they're right or not - they don't want to waste any time.
It can be frustrating, because you know why LE does what they do, because a lot of the time they are right, but when it's you and it's your loved one and you have a gut instinct and are pretty sure you're right, it's hard to know how to handle it.
Having said all this, it's been 7 years in Maura's case. If anyone is holding anything back that they felt was irrelevant at the time, it's past time to bring it up. At this point in this case nobody knows what's important and what's not, so no one should hold anything back (not saying they are). Having said all I did about my story, as much as we think we know people, no one can ever know everything about someone and what's in his or her head. This is especially true, as many others have said, of a father and a daughter in her early 20s.