3 ideas
The Cronus Club sends information back in time without any time machine. Information moves only through personal trust and memory, which makes the network very slow and easy to break if one link lies.
When Vincent brings future technology into earlier decades, events move faster. Cold War weapons and computing arrive before the institutions and moral learning that once held them in check, and each change spreads through history to bring the end nearer. The novel argues that a technology's safety depends on the society that grows up around it, not only on the invention itself.
Harry is a mnemonic, meaning he remembers every detail of every life. This lets him plan over many lives, but it also means torture or trust can pull everything out of him. Kalachakra can also be 'Forgotten', their memories wiped so they are effectively destroyed without dying. Seen this way, a person's identity is the memory that carries over, and the real way to kill an immortal is to destroy that continuity, not the body.