PLL : CeCe/Wilden/Ashley Connection THEORY
Ali said that CeCe killed Wilden and most of the reason was to help Ali. CeCe likely figured out that there was bad blood between Ashley and Wilden. If CeCe killed Wilden to help Ali, that could mean that Wilden had figured out (or knew the whole time and recently threatened to reveal) that Alison was alive. There were rumors (that I haven't seen confirmed, but could potentially make sense) that Ashley Marin knew Alison was alive before the girls did. Maybe CeCe somehow arranged (or told Ashley herself) for Ashley to find out that Alison was still alive, and that this could put Hanna in a lot of danger.
She could have filled Ashley in on the other girl who was killed and claimed that Hanna was in some way involved. We have seen CeCe spy on people before (Aria and Ezra at least), so regardless of the reason for her behavior, it's possible that CeCe spied on the girls too. She could have found out about Hanna finding out about Sara Harvey and developing her theory that it was her body in Ali's grave. If she could prove to Ashley that Hanna was looking into that girl's disappearance, that could have planted the idea in Ashley's head that Hanna has some secrets about the night THAT girl went missing.
CeCe could have known about Wilden's obsession (this was around the time before he was killed) with getting a blood sample from Hanna. CeCe could have convinced Ashley that she NEEDED to get rid of Wilden for Hanna to remain safe. Wilden was an inappropriate, creepy, rude person, but Ashley would need a very good reason to be desperate enough to want him gone that badly; she resorted to asking her ex-husband for money.
Then, we also need to consider the fact that Ashley insisted that she did NOT shoot Wilden, yet she refused to provide any actual proof of her innocence. Maybe this was because the only adequate way to prove herself innocent would have been to expose the fact that Alison was alive. If Ashley thought releasing that information would put her daughter in danger, that could explain why she wouldn't supply information that would exonerate her, and why she was willing to go to prison instead.
Wilden was friends with Jenna. If he found out that Ali was alive, there's a chance he would have told this to his blind buddy, and CeCe would have known that would put Alison's life in jeopardy.
Jenna and Shana were on the surveillance tape moving Wilden's body the night of his death. Maybe Wilden told Jenna that he had something important to tell her that night, and Jenna and Shana went to look for him when he didn't call/show up/etc. Jenna also implied that Wilden saw Ali the night she disappeared (or at least according to Garrett he did, so we can't be sure if that's true-- he lied to Jenna about killing Ali). If Jenna really did think that Garrett killed Alison, I wonder how much she thought Wilden knew about what happened that night.
Wilden freaked out when Hanna blurted out that they knew about his Cape May days on the boat with Ali and CeCe. I wonder why he was so worried if it was "just a picture." There has to be more to this. After Wilden found out that information, that's when he supposedly scared CeCe out of Rosewood, and pretty soon after that, he was dead. Maybe when Hanna told Wilden what the girls knew, it somehow triggered him to figure out (if he didn't know already) that Alison was alive, and maybe he thought Hanna knew about it, and that it could cause significant problems for him.
Also, if Wilden was able to get the gun away from Ashley, maybe he (or CeCe or someone else involved) took her phone also. If "kisses A" has anything to do with Ali and/or CeCe, putting Ashley's phone in Wilden's casket could have been a reminder that Ashley needed to keep her mouth shut, if she did in fact find out that Alison was alive. This idea is especially supported by the fact that Hanna's name in Ashley's phone was changed to "kisses." This could have been a subtle threat regarding Hanna's safety if Ashley didn't cooperate and keep quiet.
Let's also factor in Mona taking the blame for Wilden's death. If CeCe somehow had Mona believing that Ashley was trying to get rid of Wilden to protect Hanna, that could be the reason Mona confessed. Maybe her claims that she did love Hanna and that their friendship was real are true in Mona's twisted mind. She could have confessed to look like a hero in Hanna's eyes.