This is my opinion and my opinion only. Getting that journal admitted into evidence might be tough during the State's case in chief, because it is hearsay (out of court statement made to establish the truth of the facts asserted). I suspect Coangelo is going to try to get it in under the "state of mind" exception, namely to establish Jennifer's state of mind with respect to her devotion to her children and need to keep them safe.
If Dulos elects to testify, the chances of that journal coming in greatly increases IMO. Why? Because if Dulos testifies that Jennifer abandoned her children in a Gone Girl scenario, and wasn't murdered by him, that journal becomes clear rebuttal evidence to establish she was not in the "state of mind" to so callously damage the children she loved above all else. Putting Dulos on the stand is a huge risk that many defenders would not consider, given the many ways he will be impeached with his prior statements, damning evidence like this journal...and who knows what else? It seems the C Team is unconcerned with this potential for disaster, at least publicly.
The presentation of evidence in this case could be a law school exam, as there will be a great deal of it, gathered from many sources. Every state has adopted specific Rules of Evidence, which set forth what is and is not admissible in court, and these rules can be quite complex. Prosecutors usually have a better understanding of the rules of evidence than most trial lawyers, and experienced prosecutors, who have tried lots of cases, have the best grasp of all. You can be sure that Coangelo is going to be prepared to do battle to get as much evidence of guilt in as possible, and NP will do his best, whatever that is, to keep it out.