I'm not sure what to think about WN. On one hand, he seems the most obvious POI because:
1.) He had opportunity
2.) He has a history of mental illness
3.) There are too many eerie coincidences involving WN, e.g., when searchers began searching the lake, he seemed to know they would find Celina's body there; he had what appeared to be a nervous breakdown, even before the discovery was announced.
4.) LN seems to suspect him. Why else would she suddenly leave him following Celina's murder?
It may be more difficult for LE to gather DNA evidence against WN since he lived in the home and, unless body fluids were involved in the commission of the crime, his DNA would not have been out of place. The same could be said for anyone else living in that house.
If WN isn't the murderer, on the other hand, I do think he witnessed something that could solve the case, but he's too mentally unstable to remember/ express what he witnessed. Maybe he realized too late that Celina was in danger and is overwhelmed by guilt, knowing that he could have prevented her death. Maybe he is terrified of the one he knows/ suspects is responsible. I don't remember if his brother has been previously discussed as a POI:
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http://www.examiner.com/article/disturbing-new-facts-emerge-celina-cass-suspicious-death
Whether or not WN is the murderer, IMO he holds the key to solving Celina's murder. But, LE seems hesitant to pressure WN into telling what he knows due to his mentally fragile state. Could LE be concerned that all hope of solving the case would be lost if WN were driven "over the edge," so to speak? I do know that Celina needs justice and until justice is served, there is the danger that the culprit will strike again.