I’m not a lawyer, but it seems like the defense of unconstitutionality would be a claim that bringing an impeachment charge of any kind (including “incitement of insurrection”) against a former president is not permitted under the Constitution.
But if there is no challenge to the constitutionality of the articles of impeachment, that still doesn’t make evidence of election fraud relevant in this case. On January 8 President Trump said “To demonstrators who infiltrated the Capitol: you have defiled the seat of American democracy. To those who engage in the acts of violence and destruction: you do not represent our country. And to those who broke the law: you will pay.” So his defense is that he is innocent of inciting insurrection. He did not want it, although he was in favor of pursuing all legal means to expose the truth. Whether the truth was successfully exposed or not is not relevant to whether or not President Trump incited insurrection. He simply did not.
Personally I wish that all the thousands of people who went to D.C. on 1-6 could send a statement claiming not guilty of insurrection. They were there to give support to their President and to let America know that they will continue to support him. Who would they send such statements?