Cliff the following diagram is from the SkepticalScience article we went over before.
The coefficient of expansion decreaes with depth (pressure) to about 1000 meters, which is where the heat would be relocated with OTEC using a deep water condenser.
With OTEC you typically loose 1/4 of the delta T through the evaporator, 1/2 through the turbine and 1/4 through the condenser. With delta T of 20 you would lose about 5 degrees at depth. From your reference which only shows a depth of 2000 meters raising the temperature from 5 degrees (actually around 4) to 10 would raise the coefficient of expansion from from 157 to 202 which is still less than 257.
Regarding radiative cooling OTEC would be removing surface heat 24/7 while the surface would only be warming during the day.
As to plumes and entrainment I believe the deep water condenser overcomes most of this as does James Lau's design which requires no water movement due to the dispersed nature of the evaporators and condensers.