I have been accepted to an event where a keynote lecturer is a plagiarist
What really matters here is what your goal is. Is your goal to: (A) cause there to be some sort of disciplinary action/reaction against this person; or (B) to make sure that your own name is not tarnished by association with this person in the future. 

If you are hoping for (A) I believe that you have answered your own question. It does not appear that any action will be taken against this person based on anything you do in relation to this conference. The discipline is covering this up and everyone is going along with it and as an early career researcher, you are not influential enough to change this. It's sad, but it's true. However (B) is entirely possible for you. If you believe that one day this person will be publicly shamed and denounced fully by the discipline, it is not unreasonable for you to not want your name associated with theirs. This might mean withdrawing from the conference and specifically asking that your name be removed from any material online about the event. Indeed, you don't even need to say why you are pulling out of the event if that includes a risk that it will get back to the person in question. 

In this situation, I think (B) is a reasonable concern and a reasonable reason to not speak at a conference. I also think that you are tangentially associated with this person so are unlikely to experience any negatives from speaking after them at the event. Ultimately, with risk of association low, it comes down to your own ethical stance and if you feel sharing a stage with this person violates that.