After I asked the question, I walked away from the computer for awhile. I was so sickened by the revelation of the heart sticker that I couldn't fathom how it could be used to disprove guilt. I took a short walk in the cold and put on my lawyer hat. (No, I'm not a lawyer, but I think like one.)
It could be argued that attaching the sticker shows lack of planning and deliberation. Rather, it's a rash and reckless act. As for malice aforethought, the general public has a hard time accepting that a mother could kill her child. Just look at the number of doubters on this board. Fillicide and infanticide are such unnatural acts, we can't wrap our heads around it. To convince a jury, the prosecutors will attempt to portray a vile selfish woman with deep resentment and hatred for her child. But the sticker shows love, not to mention all the happy photos. The prosecution will have to show that she both loved and hated Caylee, which will confuse the jury. So, I agree. It is a potential obstacle. I don't like it, but I agree.