late stage tech halt
Local supernova takes out electronics and power supplies
the reason for the Fermi Paradox shows up, most likely the alien AI simply launches a hypervelocity object at the solar system. Good scifi story on that already.
friendly AI halt could still cause existential risk in systems less than nuclear, if they are controlling all bots...
It seems nano-toxicity is also about the ability to cross the blood/brain barrier. If some particles trigger the same toxicity dampening effects that the Alzheimers plaques try to implement, inflammation will continue to damage the brain and it’s electrochem interactions. Brain as grey-goo
http://www.alz.org/braintour/plaques.asp
it also seems like you haven’t included prions in your graph. A prion in the wild is a possible template for self replication of another protein, that may have horrid outcomes. It is also pretty simple to see distribution in the food network, perhaps as a food color, as an easy, and accidental distribution method
edit Seems like you haven’t read Ringworld, or you would have some worse considerations of a specific carbon cutting organism released to the atmosphere. For instance, what if something was developed that only cut the O-O bond , in particular, that CO2 would get a lot deadlier....