She joked about stealing Zoe's boyfriend, and she has every right to. She should make Zoe eat a massive chunk of humble pie. Every day. For the rest of her life. Zoe should kiss Stacey's shoes and worship Princess Stacey for eternity.
Stacey volunteered, yes, but Kat let her volunteer
without telling her the whole situation, making Stacey think it was just a little white lie.
The reasons for Stacey protecting Zoe stem from Stacey's disastrous relationship with her mother, and her initial "black sheep" status in the Slater family.
Stacey's mother rejected her, as did the Slater family at first. Only Ruby, Dennis, and Nana Moon accepted Stacey as she i, unconditionally. Kat has become a mother figure to Stacey, and thus has alot of power over her. Kat does genuinely love Stacey, but Kat is also by her nature manipulative. By the time Kat had realised how much she had hurt Stacey, it was too late, her attempt to resolve the matter failed because Stacey was by that point too far gone.
Stacey is very different from Kat in alot of ways, being outward looking whereas Kat tends to have a narrow focus (Zoe! Zoe! Zoe!)
If Zoe went to prison, Kat would be devastated and Stacey would blame herself. That's why Stacey kept saying about "letting them down" and "they'll hate me"...she fears being rejected and unloved, and craves acceptance and being part of a family again. Big Mo's comment about her "coming through for us" sadly confirms Stacey's perception. She is actually a very vulnerable girl, despite her formidable strength. Think about it and Stacey's actions make perfect sense.
Remember the Kobiashi Maru (no-win scenario/Catch 22) in Star Trek Wrath of Khan? That was Stacey's situation.
Stacey's relationship with her mother is tackled in a storyline (and it is Stacey's first singular storyline) coming in early December.
It's entirely feasible that Chrissie has said she slipped out that night and returned without Stacey knowing. After all, Stacey is a teenager. Remember Kat made Chrissie promise.
If there is anyone who should get away with it, it is Stacey. Her motives were honourable if misguided, and, unlike many of the oother people in this storyline, she felt for people outside her own family, and was not acting selfishly.
Besides EE Producer's can ill afford to have Stacey locked up for even a few weeks, as the character and the actress who play her are indispensible to the show (and after Kat Alfie etc leave they will need her even more).. EE without Stacey is tedious.