It's kind of a toss-up for me, but I think the light is something of a distraction for other animals, more than a way for them to see. Let's keep the darkvision.
Is the name too confusing, implying they are animals? Perhaps call them "Depth Dwellers" instead?
Suggested DC for the Spot check in the ink cloud ability?
We could go either way with the name. What do you think, Cleon?
DC 20 to let them get away?
For the tentacle severing: 2 hp per arm, no damage to the creature?
Mmmph, I see what you're saying about the ink, but I really don't like Cha-based for an Ex ability. Just feels wrong somehow
Max Spot, I think.
Stealthy, either Alertness or Skill Focus (Spot)?
Deep-dwelling octopi have established an agrarian society. Near the hottest land most nutrient-rich) part of a vent, they raise giant tube worms and clams. The clams and worms provide food and, when the shells are cleaned out, shelter. Farther from a vent, the octopoids tend fields of shrimp and mussels. Some octopoid communities are known to raise crabs and lobsters as well.
Deep-dwellers do not have a concept of money as such, but individuals may possess treasure. Pearls are often strung to form necklaces or bracelets, then sprayed with ink so as to glow. Octopoids are fine sculptors whose work would certainly be considered unique if brought to the surface.
A common ploy is for the octopi to approach their victims from above, having "turned off" their luminescence. In such cases characters take a -2 penalty to their surprise rolls.