This case is unlike most criminal cases. In most cases, there is no question a crime was committed, rather it is only whether the Defendant did it. Or, as in a rape case, the Defense may deny that the sex was rape, that no crime occurred.
In this case, the Defense is claiming a crime did occur, but that TM committed it. The prosecution claims no, GZ committed the crime. This isn't a "who dunnit," but a "which one dunnit." Or did either of them commit a crime at all and this a traged of errors.
That's why I say if the door is opened to GZ's past, it must also open up on TM's past.
A real question is whether or not the full tapes get into evidence. If they do, GZ has told his story without taking the stand.
EVEN IF GZ took the stand, I really doubt the court would allow hardly anything from his distant past UNLESS GZ opened the door by saying "I've never hurt anyone," "I always tell the truth" etc. I really don't think the court is going to allow tagging in a sexual assault case against a minor a decade or more old, certainly not a civil divorce suit restraining order issue, and it would likely be appealable error if the court allowed evidence of an arrest that did not lead to conviction.
Usually, not always, courts MUST draw a tight border around relevancy, or trials would last forever. The question is did GZ murder TM or not? Not whether GZ is a lying, creepy asshole or a really great cool guy.
If the prosecution can open that door, it might backfire like it did in the Casey Anthony case. It can make the prosecutor seem desperate and trying to get around a lack of evidence. I seem to recall some of the Casey Anthony jurors saw it that way when the prosecutor put on bizarre and questionable evidence and theories. Doing so also opens up the barn door to the defense and gives almost countless basis pursue an appeal.