Trump's recorded conversation with Raffensperger asking for more votes is a key piece of the prosecution's case. It can't be admitted without allowing the defense to cross examine.
Cross-examination is not an invitation to a fishing expedition. On cross, the examining attorney is generally limited to the subjects that were discussed on direct, as well as issues going to the credibility of the witness. Georgia state rules on examination are broader than the federal rule, inasmuch as they provide that "[a ] witness may be cross-examined on any matter relevant to any issue in the proceeding." Ga. Code Ann. §24-6-611. However, that process is still subject to overall control by the judge, who is specifically directed to control examination to (1) "Make the interrogation and presentation effective for the ascertainment of the truth", (2) "Avoid needless consumption of time;" and (3) "Protect witnesses from harassment or undue embarrassment." Id.
There will be precious little room to open a free-ranging inquiry into what Raffensperger did, particularly on cross-examination.
I would fully expect the prosecution to make a motion in limine to ring-fence that sort of inquiry.
