If you do try to add in some new invasive indigenous species just remember to add them to the deep sea level, we have less species variety when compared to the sea of the shallow environment. :-(
Anyways when talking about food web, I think that "?" monster is already is figured out because on the first turn my species encountered a deep sea salamander that ate of the Stubbornians and feeling disgruntled afterwards. Also some of the creatures on the higher tier of the food chain also eat off of the plants as well as having prey for them. Essentially, for example does my species of Stubbornians digest things like Deep Sea Crab and the detritus/ocean snow with its dredging methods. That's it really to address with the Food web, looks good.
Finally you said that the tentacles of my creatures couldn't have thorns on their tentacles because it would interfere with the venom extrusion, how about this, if my species adapts to have two sets of tentacles. Their would be two layers of tentacles; the outer layer of tentacles would be the non-venomous kind that would have sharp thorns around it to grab the prey, and have would even have more tensile strength to make swimming more efficient. The inner layer of tentacles would be covered in the cilia-hair and have the glands for the corrosive venom for the devouring and killing of the prey. How much EP would this cost; 3?, 4?
One other note I'm pretty sure you would have 22 members of your species for the Mychords at the beginning because the starting population equation is 2(100/(3x3), not 33 (Maybe later players get initial bonus, I don't know)