You know, this makes a lot of sense. Like some sort of peeping tom or serial rapist, the kind who literally jumps out of the bushes or climbs in people's windows. An actual boogey man, of sorts. Reminds me of the Brianna Denison case.
The more I think about it, the more I agree it wouldn't have been someone close to the family unless Clint was in on it (which I don't believe he was), because they would've known he'd be home, and why risk it?
The only question I have about your theory, Jeannie, is that I guess I didn't realize Clint ever showed his face. Do we know that the perp saw Clint or knew Clint saw him?
Of course, even if we did know for certain that Clint scared him away, that doesn't tell us much other than it can't be someone who is TOO close to the family. Going by the theory that he took her and ran because he saw Clint, yes it could be a stranger, but it could also be a person who knew Clint could have identified him.
Another thought - not that this part matters much, but just because his plan may have been thwarted by discovering that Clint was home (if he did), we don't know that he only meant to assault her but not kill her. He very well could have planned to do what most sexual predators do and assault her AND kill her, even if Clint wasn't home. Serial rapists who only rape (and I'm just using rape because, well, what else do they usually do?) are pretty rare in comparison to those who also kill their victims. Who knows?