ok from my understanding....
regular fan does not come on..fan switch
lower hose is cooler than upper hose..thermostat
but if the thermostat is not opening and allowing coolant to pass through, then the fan switch cannot read temp of coolant, therefore not allowing the fan to come on, correct?
i had the same problem on an ls integra i worked on not to long ago..same syptoms, ended up being the thermostat was actually bent and getting stuck half way while opening...just my 2 cents...hope that helps