I think when striking the plea deal it was probably done after LE could already prove that she wasn't there when KB was murdered. Cell pings, Survelience video they had of PF coming and going etc.
I think It's very probable that LE told her listen we know you wern't there when KB was murdered we will give you this deal.
You have no reason to not be truthful and if we find that you are not being truthful we will charge you accordingly and void the plea.
KK would have no reason to outright lie at this point as she would be risking possible LWOP or min 20yrs, when if she just tells the truth she gets a slap on the wrist.
Why would she continue to lie?
I don't think she would tell a lie. I think she might omit some of the truth that might be helpful.
For example, PF might have said he was going to dispose of KB's remains at the landfill or a river. He might have given her more specific information about their disposal. If neither she or PF talk about it, LE will never know.
Technically, I guess that would be withholding information or a lie by omission.