In this alternate earth, during the Permian period, a sentient aquatic cephalopod species evolved intelligence equal if not superior to humans. Unfortunately, there's a big problem to a cephalopod species becoming advanced enough to have a society. This is because when cephalopods breed a hormonal kill switch is activated, turning off the digestive system and hyper-charging the reproductive system. To get past this constraint the species and its ancestry would have to gut its entire internal system and reproductive strategy, and rework it into something more like that of chordates, which would be hard to overcome because...well it works. There are a few exceptions such as the chambered nautiluses, but they are relatively recent evolutions, appearing in the fossil record 55mya after the KT extinction. I'm asking for ways to allow my cephalopod to survive past the automatic "kill-switch" and live for many years! (This idea is heavily inspired by an answer to another question! 😁)
My extensive (5 minute) look into the subject says that 1: this is hormonal and 2: removal of the optic gland prevents the self-destructive behavior (I didn't find anything related to the digestive tract?).
So the “simple” solution: some cephalopods had a very very rare genetic disorder that causes the optic glands to either kill themselves or suddenly ramp down its hormonal production for a few months after it's done it hormonal sex-overdrive. That would mean that a single cephalopod could have multiple offspring periods in a single life and potentially use its intelligence to support their children in their life. These would eventually start dominating the gene pool.
The best case scenario, assuming that the entire self-destructive process is a boon for the kids, is that the process stops just short of killing the cepholapod, after which it can recuperate.
As you point out, the ability to have multiple breeding cycles did evolve in cephalopods, with the vampire squid being another example so there's no particular reason why that could have not evolved sooner. Indeed, it's somewhat unlikely that the largest orthocones like Cameroceras could live long enough to grow shells several meters long and only breed once.
A favorable mutation that allows for a female octopus to survive guarding her first clutch of eggs would likely do the trick. Octopus don’t spawn multiple times in their lives but simply produce so many at once that they can be sure their genes will spread (an R strategy). But if a female octopus was able to survive one spawning and produce another, that would be twice the number of living descendants with her favorable gene. The gene for multiple spawning would spread as it has a selective advantage. The repeated spawnings would select for longer life, as the longer lifespans would allow for more spawnings and therefore more genetic descendants.
A neuter cephalopod, which is a mutation of any kind that prevents it from breeding.
Your species produces male, female, and neuter. The neuter ones are the intelligent ones, they can recognize their siblings, and proceed to guard, nurture, and breed them. This cephalopod does not die after breeding because it does not breeding, and its ability to protect its family allow it to flourish.
Nautiluses (nautili?) are awesome. Maybe they are not as smart as octopi but they are clearly cephalopods and they don't die when they lay eggs.
Nautiluses reproduce by laying eggs. Gravid females attach the fertilized eggs, either singly or in small batches, to rocks in warmer waters (21-25 Celsius), whereupon the eggs take eight to twelve months to develop until the 30-millimetre (1.2 in) juveniles hatch. Females spawn once per year and regenerate their gonads, making nautiluses the only cephalopods to present iteroparity or polycyclic spawning.
The short lifespan of higher cephalopods is interesting. Maybe their Permian ancestors hacked their genomes to die young before intelligence could cause trouble - like replicants in Blade Runner. By trouble I mean trouble like intelligent entities ultimately bringing about their own extinction.
Your entities back in the day performed an additional hack rendering them functionally immortal, but also total cooperative sweetie pies.
It seems a hormonal kill switch is the only limitation. Then the cephalopods can regulate this with with something similar to the pill in humans. The pill also regulates the hormones, allowing you to stave off pregnancy. In cephalopods it might simply remove the kill switch part after the pregnancy. As an alternative to what you are asking, it can also stave off the pregnancy. They still won't live past their kill switch in this alternative, but can extend their life.
As an aside, improving the life of a cephalopod could stave off the time they decide to reproduce. Just like in humans, where high welfare causes people to get kids at a later time, their trigger to reproduce might also be delayed. Some cephalopods in captivity can live much longer lives.
how to get the pill
There are two ways to get this pill. One is batural, the other is scientific.
For nature it can be simple. A plant or other organism has a defence mechanism that poisons the attacker. Instead of killing the attacker, it delays the reproductive cycle for at least the cephalopods. It can be that many other organisms aren't as lucky as your cephalopods and die/become sterile. In normal circumstances even a delay of reproduction is advantageous for the plant or organism, as less cephalopods will survive to reproduction. This in turn increases the chance of the plant or organism to spread, or at least other plants or organisms to thrive. However, if the cephalopods are doing well this can be used to extend the life span. It can be used as birth control and can potentially stop the kill switch, even after spawning.
This is not unlike so many plants with defence mechanisms. They can be hallucinogenic, make you feel good or be the spices humans use for food. Most are actually defence mechanisms that we can now use as recreation or flavour.
Scientific is more difficult. The cephalopods need to have a long enough life cycle to learn and pass on knowledge to the next generation. You need chemistry if the ingredients aren't easily found in nature to make it. The scientific method is difficult, but not impossible. However, why write about a near impossibly, while a plausible alternative of natural occurrence is available?
Immune System Mutation
. . . when cephalopods breed a hormonal kill switch is activated, turning off the digestive system and hyper-charging the reproductive system.
The blood is full of white blood cells. The squids mutated so the white blood cells now attack the kill switch hormone. It is still produced but it never gets where it needs to go.
This is similar to real autoimmune diseases, where your immune system attacks some useful part of your body. White cells attack the platelets? You have a blood-clotting immunological disease. White cells attack the red cells? You have chronic fatigue. White cells attack each other? You have something like AIDS.
These diseases are often genetic. You can have something even if neither parent has it. But they both carry the gene. Some time millions of year ago this gene was created by a bad genetic fluctuation. The squids did the same, only their fluctuation was good.