It would be highly unusual for a witness who isn't privy to the computer forensic results, isn't privy to the VOIP or phone analysis, isn't privy to any physical evidence to suddenly become THE foundation that proves murder. She is one link in a chain--she is one witness who is providing context about the victim's state of mind, the general state of things during different time periods, she is providing information about the victim and to a lesser extent, some behavior/anecdotal info about the defendant. This witness is not intended to be the AHA! witness. Nor could she be.
I truly think we're going to have to wait for the ENTIRE prosecution case to be revealed to get the full picture. It may not just be ONE thing that provides proof...it may be a bunch of smaller things that when added up are hard (if not nearly impossible) to explain away as mere coinky-dink.
And yes, you will see that the prosecution asks every one of Nancy's friends if they intended their emails to be sent to Brad. Why? Because Brad was SECRETLY getting a copy of every email sent to Nancy. The way they establish this fact is to bring up the intent of each sender of email, confirm that they only sent it to Nancy, and then later through a computer forensic examiner witness prove that Brad was accessing Nancy's email secretly!