Was it just me or did it seem like Grundy/Jennifer actually wanted to leave town when they found out about her real name? like she genuinely looked shocked when Archie confronted her with that. Think about it: what if Grundy knew that Jason was gonna get murdered, or she did it herself, whatever. She would need someone to back her up, be her alibi and she would need an excuse to skip town.
What if Grundy wanted the relationship she had with Archie to be discovered after the murder so that she could have a valid reason to leave town, making it seem like she had no choice. Perhaps she thought that being caught in such a scandal was better than being caught with murder.
And if she played her cards right, she wouldn’t get in serious trouble. All she had to do was pretend to be a victim of sexual assault. Blame it on that. She could argue that after such a horrid and violent relationship she wasn’t used to Archies kindness, she felt cherished, she was vulnerable. The person that would find about the relationship would feel sorry for her and make her leave town as expected. The perfect excuse for her to leave without it looking like she left because of what she did to Jason.
However, her plan failed as she never expected B & V to find out her real name and now they know a part of a much bigger secret. So now she has to fix that.