Let us think of body plans. The main issue with living on land is not drying out. Did any relatives of the octopi manage that?
Depicted: snail, nautilus (both from Wikipedia)
Snails are second cousins to the cephalopods. They have a mollusc body plan and protect themselves against harsh circumstances on land with their shell. The shell allows water conservation and also protection from heat. Snails can survive even in deserts.
So too your land-going octopoids. They have robust shells to protect them. They make slime to get around. There are existing cephalopods with shells - the nautili. They are cool beasts. Use them as your model.
I was thinking your land would be brutal for land-dwellers but realized I was thinking of the ocean as our own cool pH neutral ocean. Of course in such a hot planet the ocean will be hot, and an atmosphere with so much SO2 will mean the ocean is sulfuric acid. All good!
As to why they would go on land, why does any immigrant emigrate? Escaping bad circumstances at place of origin, or taking advantage of good opportunities in new place. Your ancient pioneers could leave the water and find food on the shore. It was safe from aquatic predators on the shore and such predators as there were on land were not able to prey on your octopi. Evolution over time selected for individuals who could spend more time out of the water and on the beach and points inland. That is how it worked in our world, several times.
Thinking about this does make me curious about the evolution of snails (and slugs!) as compared to other terrestrial lineages. I will need to read up.