How can a plaintiff avoid being faced with unfair prejudice after a defendant does a motion to strike all evidence from a dismissed PO? Since without the evidence the plaintiff can't defend their actions for the dismissed PO, wouldn't that be unfair prejudice? They would automatically be guilty of something in the eyes of the judge. Can rule 403 allow the plaintiff to ask the judge to strike the judgement of the dismissed PO too if the evidence on it is not allowed?
The order of operations is important
I assume that plaintiff filed for a Protective Order. To get this granted, the plaintiff has to allege some kind of wrongdoing and evidence of that.
If the defendant responds, then the plaintiff can amend their filing. Then the defendant once more can respond to the allegations.
If the plaintiff wants to amend the filing once more, they need to ask the court to be allowed to do so, and that opens the door for the defendant to answer once more.
That's all history for the case presented: The court apparently found the evidence lacking and dismissed the application for a PO. Plaintiff can only file for reconsideration or appeal but not bring in new evidence at this point.
Dismissed Cases are not automatically evidence
A case that did not establish its burden of proof and was dismissed - especially with prejudice - has not established that the evidence in it is good. You have to ask each item to be admitted separately and re-establish that it is good evidence.
A bulk filing "I want to bring this case as evidence" is generally denied unless you prevailed in that case. A dismissed case is one you didn't prevail in.
Get a Lawyer!
It seems like you are in serious need of legal counsel to clear up the situation. Contact a lawyer for at least a free consultation if you even have a case.