We have to assume it was Charlotte, not Mona. Although Mona claims that she was with Emily at the tomb, we only see Mona walk out of Radley using a nurse outfit in 3x11, with an Ali Dee badge. Speculate away.

Again, Charlotte.

Shana seems to be Jenna’s A, let me elaborate. Just like Toby was an A-minion/spy for Spencer, Ezra was an A-like entity for Aria, and Mona’s motives for being A seemed to be Hanna, Shana was Jenna’s A. Someone who was poking around, trying to keep Jenna safe.

No, Ian killed himself and Mona found his body/phone/NAT videos. It is unclear why Ian committed suicide though, because the police weren't believing the girls, and it seemed he had everyone on his side. It’s possible that we'll find out in the end.

Child pornography is a big crime. The NAT Club is no petty misdemeanor. They didn't want to be involved, because at the age that they were doing those videos, they were 20ish, they weren't just perverts, they would be branded sexual offenders, and rightfully so. Jason included.

Refer to the 5th. His confrontation with Alison that night might have left trace vidence on her, residue of hair, etc. Since Jenna had a scuffle with Alison, he was protecting her. Also, an investigation on them would bring consequences such as the NAT videos being uncovered.

We don't have a lot to work with when it comes to Wilden, but he was a super corrupt cop who loved taking advantage of vulnerable older woman, such as Ashley, or as Pam Fields when she was asking Barry a question, then he interrupted to be all flirtatious and weird. Aria found basically only a stack of porn in his house, no trinkets or sentimental items, just porn. We can assume that he didn't have much depth. Charlotte might have killed him, but she never completely admited to do so, and Marlene has said that "CeCe" killed Wilden, even though she answered other questions with Charlotte.