The grid sales seems to be reflected in the COE. If so, is COE a better indicator of feasibility than NPC?

Grid sales income affects both NPC and COE.  You might be noticing that they affect the COE more than the NPC, but that's just because grid sales both reduce the numerator (total annualized cost) and increase the denominator (annual useful electrical production) in the COE equation.  I think the NPC is always a better indicator of feasibility than the COE.  A FAQ in the help file explains why.