The choice that the jury had was a)did an intruder "murder" the boys and what is the evidence that supports that and b)did Darlie murder the boys and what evidence is there to support that.
If you have no evidence that an intruder entered the house and murdered the boys, then you have no choice but to outright reject that "defense". If you find that there is ample evidence that Darlie murdered the boys then she is conivcted.
How could Mulder "provide" evidence of an intruder when there is none. Nothing. Zip. Nada. But the DA had ample evidence shown at trial that Darlie did murder the two boys. So you can present a "defence" but if you have nothing to convince the jury, then they have nothing to go on.
Some Lawyers can come up "with creative" defenses because it their obligation to "defend" their client, that does not mean that the defense will be believed or the defence is credible, but it is after all a a defence.
So to say that "Mulder" offered no defense is just not accurate, he tried to refute the evidence against Darlie, he tried the "intruder" card, he tried and tried, but again no evidence to support his defence "theory" of an intruder.
Many, many "convicted" people are in prison because they testified on the stand. They somehow "think" that they are smarter and "more" convincing then they are given credit for. But they fail to see how they "come off" when testifying, they are subject to cross, questions they could not prepare for, stories that are not credible, with "I can't remember" I don't know. A lawyers "worst nightmare" is having a client testify. It is as if, all the hard work a lawyer has done is "undone" by his client wanting to "give evidence" on the stand.
All a person can do is sit back and "cringe". But if the client against advice gives evidence, then the lawyer cannot prevent them from doing so. After all they are the "client" and the person with the law degree and experience is "just the lawyer". The client "directs" the lawyer and the lawyer "advises" the client, if the client heeds the advice of the lawyer is a whole different story.