2001 LeSabre, possible oil leak... questions

Actually I'm pretty sure I have oil AND coolant leaking. There's a mess of greasy watery nastiness in the depressions around the front injectors, between the plastic top of the intake and the metal cover part that has the oil filler in it. Looks like the seams around the nearby driver-side coolant conduit (which has one of the radiator hoses connected to it and which I'm assuming contains the thermostat) may be leaking fluid. Plus, the guys at autozone pulled a PO128 code, which states:

"The PCM has determined that the input from the coolant temperature sensor indicates a temperature value that is below the thermostat regulating temperature."

Since I have also been having heater issues, I am going to first try replacing the thermostat, and the thermostat gasket of course. If this doesn't solve the code issue, I'll replace the coolant temp sensor. Once that problem is cleared up, my potential oil leak should be easier to locate.

My first question is, is the coolant temperature sensor the sensor that plugs in directly below the thermostat?

If there is an oil leak, it almost seems like it's coming from the seal between the upper plastic part on the intake and then rest of the engine, though that doesn't make much sense to me. Some upper gasket, anyhow... which leads me to my next question, which is: if I do determine that I have an oil leak, can anyone briefly tell me how difficult and expensive upper gasket replacement can be on this car? (I'm only knowledgable of rotary engines so please forgive my piston ignorance!)

Thank you!

Yes it is a 2 or 3 wire sensor. Wash the engine off good (Brake clean not water) and start from scratch to track the source of the oil leak, these engines are not known for oil leak problems generally.