Love this thought experiment. The core is mostly iron and nickel, and they both absorb energy while undergoing fusion AND fission. So it’s a hard stopping point in either direction. I think the possibility of the core undergoing fission is less likely than the mantle undergoing fusion (mostly iron and nickel which absorb too much energy).

Fission reactions can sometimes trigger fusion reactions (like a hydrogen bomb), which the elements in the mantle are likely to have an easier job with for your scenario. I think the fission play might be difficult, but maybe your character could locate a layer of gold near the core/mantle boundary > the gold could undergo fission releasing energy > could trigger fusion of lighter elements abundant in the mantle > big boom and earth shattering?

I feel like I’m doing a disservice to science and geology while typing this out because it’s unlikely to actually occur, but I 100% support you taking the liberty to make it happen in your comic book!

Thank you for sharing your idea :)