1) If uniformitarianism and naturalism are anti-biblical assumptions as ways to explain things in the past, how can he use the fact that a canyon formed 30 years ago as an argument for his position?
What I read Ken Ham to be saying is a tautology we'd all agree with. Science
is anti-biblical to the extent that it contradicts the Bible. Of course Ken does not even entertain the idea that either the Bible or his interpretation of the Bible could be wrong.
I'd also submit that the Saint Helen's explanation does not have to be true for the argument to have some merit. The only point is only that canyon formation does not take millions or billions of years even using natural processes and not that God necessarily used a volcano to create the Grand Canyon.
It should be pretty clear that the idea that the Grand Canyon was formed by the Flood or that humans partied with Stegosauri is no more Biblical than anything Ken Ham decries.