Welcome!! And great first post!
I completely agree with you. I think that, if we are to believe WH to some extent, the person who has been interrogated multiple times and who concealed her jacket is clearly the prime suspect. Even if it wasn't that POI, it sounds like the police still have at least two more solid leads to look into. Although it seems like little is being done, I would imagine it is quite the opposite.
I've said this before, but I think we might be at a point where we are waiting on DNA results (from the jacket, the credit card, the car, etc...) or some other forensic evidence. In my opinion I think this case is probably very nearly closed and the LE really doesn't have anything to say until it is.
I do wonder, however, why there do not appear to be physical searches for her going on. I mean, if this 4-time interview guy is the main POI, surely they've investigated ties he has to the area, woods/water locations he knows, etc. Perhaps even that is all behind the scenes.