Due diligence usually refers to the review of the asset you're trying to acquire. There weren't any significant surprises about Twitter's state. Elon's issue was Elon.
Elon specifically waived the ability to withdraw because of post-signing due diligence in his offer to twitter's board. It's one of the (many) reasons he had little ability to get out of the deal once he changed his mind.
As I just argued, due diligence wouldn't have uncovered any surprises. It's merely that Elon could have used the process to ascertain what consequences the acquisition would have for his own assets (compatibility/marketing audit) and what his ideas would lead to.
It potentially could have. For example, post-signing he had a big fight with twitter's lawyers over the right way to measure bot accounts. If he could prove it was much higher than twitter's SEC filings, he might have been able to withdraw from the deal if he kept the due diligence condition.