In a "backwards kinda way" this might be an asset to the defense. After all, I don't think even they believe they have a snowball's chance of getting her acquitted. This is simply about saving her life or being convicted of a lesser charge. But think of the impression she'll make on a jury. I have severe doubts anyone will buy any accidental death story that comes flying out of her mouth, but maybe one or two will. I have severe doubts that, even if they do, anyone would buy a story about her being "petrified" of telling her parents and partying for 31 days as if nothing had happened. We've all seen her manipulate them with her self-righteous anger whenever questioned about anything, or even being arrested for that matter. She certainly wasn't afraid to stay with her parents for a couple of months when she bonded out - or to scream at her dad to "start being more of a dad and less of a cop" when he asked her for an explanation. So that would really be a dumb road to go down, making her seem afraid of them.
The only possible thing I can think of is that quote by JG a long time ago that basically said he felt that, whatever happened to Caylee, Casey had thought up a nicer story in her head and totally believed it. If she comes up with a ridiculous story - even one that is unbelievable - and she comes across to a jury as someone who is delusional to believe her own lies, that might give them pause. Not necessarily about her guilt, but about the fact that she in no way believes herself guilty of a crime. Even if that doesn't fall within the parameters of insanity, it may make a jury think there is something definitely wrong with her and that might just save her from the DP.
I know the jury will believe she is a total pathological liar, but if they come away with the impression she completely believes her own lies, it might give them pause. Even if they realize they are not hearing the truth, it may introduce a reasonable enough doubt for them to go with a lesser charge because there is simply no way to ever get the truth out of her - she "can't handle the truth" for whatever reason.
Of course ICA is going to think she is a star and is charming everyone. She's a flaming narcissist. But perhaps this is a reverse-engineered version of Primal Fear. The defendant there fooled the jury and his attorney into thinking he was a multiple personality. If the DT puts her on the stand, they may be hoping that, whatever she feels she is accomplishing, a jury is going to hear her nonsense and might think she is absolutely fundamentally flawed and delusional in a way that simply defies definition. I don't think AF was using the terms "diminished capacity" for nothing.
We have seen sociopaths like Diane Downs and Ted Bundy convicted, thank goodness. And it's still easy to pass off people with no apparent conscience as "evil" - heaven knows the consequences of their actions are horrific. A lot of sociopaths never get to the point where they commit a capital crime - but the corporate world is full of many who probably should be incarcerated, as is Bernie Madoff. We are also entering an era where, with fMRI brain scans and other neurophysiological advances, we are learning that people born with brains that are fundamentally different and who process information differently, while completely responsible for their actions, may not be completely responsible for their conditions.
This girl can't manage to deal with the gravity of her situation for five minutes - couldn't manage to even fake being interested in her daughter's death. Only an absolute moron would have failed to notice it might be important to feign interest in a "missing" daughter. I have no doubt she plied JB and his team with elaborate lies the last three years. Managing her behavior is really getting to them all - JB practically stopped visiting her last year and both DS and AF look like they are humoring a character out of Willie Wonka - one that might throw a fit or turn into a giant blueberry. I think they realize she is batpoop crazy. Not in any kind of way that would qualify legally for insanity. But maybe they want to show the jury just what it's like to be on Planet Casey and have them look at her like some absurd Great White Shark that does not deserve to be killed even if she definitely deserves to be safely contained. I really don't know any other reason to even have her up there. There isn't anything normal about her and maybe they just want people to see how totally out-there she really is. Mercy based on that kind of doubt is about her only shot at staying alive otherwise I imagine.