Quantum immortality seemed to work when I was imagining my consciousness as a thread running through the many worlds, one that couldn't possibly enter a world where I was dead. But if I understand rightly, consciousness is not like this, it is not epiphenomenal, it is not a thread that runs through one world and not the others, it is splitting along with the world around me and the rest of my body.

Right, it's less like a thread and more like a tree.

So if I undergo the classic 50/50 decaying radioactive particle + gun experiment, it would seem to me that I have a 50% chance of my consciousness surviving and a 50% of it going *ping* out of existence when the bullet pulverises my brain.

I don't understand this at all (if we're assuming MWI is true). If MWI is true, a person survives (or rather, has something as good as survival) by having "successors" - that is, beings who remember being him.

In the 50/50 case, he has half as many successors as he would normally have. But it's not obvious why this should really trouble him (aside from knock-on effects on his loved ones in the half of existence where he dies, etc).