If he doesn't show up, Biden should call Trump direct on his cell phone with a spoofed number from one of Trump's associates, and then ask him live on air why he's not at the debate, and offer to have the debate right then and there with Trump on speaker.
If you're gonna be in charge of the USA's intelligence apparatus, and your opponent is okay with weaponizing the power of that exact same apparatus against you when they feel like it, I think you should be allowed to flex with it just a little bit from time to time.