I believe the only issues that can come up during the impeachment are what he is being charged with: inciting a riot, or violence, or an insurrection, or whatever the hell they said. However, one of the issues raised in the article of impeachment was that he was spreading rumors about the fraudulent election. So, because THEY brought up the election results, he can present his proof (video, mathematical analysis, etc.).

As for the pedo and corruption stuff, I'm not sure he can present any of that because it has nothing to do with his case.

Just my thoughts.