my consciousness observes exactly one world at once
On macroscopic scale, yes. Trying to observe a particle in the double-slit experiment could change your mind (you might realize you are observing an interaction of multiple worlds, differing only in the trajectory of the observed particle). This is probably irrelevant for everyday life, but using the word "quantum" reminds people that it's at least technically not true.
It could possibly become more relevant in a very far future, approaching the heat death of the universe, if we take the immortality literally.
my argument is that Quantum Immortality works backward in time, if that makes sense.
Seems to me like an example of selection bias. Some of your future you's will die, and the rest of them will happily exclaim: "I knew I was immortal!" The question is whether this is a "correct" way to describe reality (and what specifically "correct" means in this context).
The "Wigner's Friend" experiment has some interesting examples that physicists already thought about.
whether this is a "correct" way to describe reality
I'll find out in about 100 years.