There were two different motors which were used in this model. One was a 3.0 liter SOHC V-6, model 6G72 (VIN code H). The other motor is a 3.5 liter SOHC V-6, model 6G74 (VIN code R). The two motors used different spark plugs; so if the engine type was not specified when selecting plugs; it may be that the wrong spark plugs were installed.
If the # 2 plug is misfiring; this sounds like either the compression in that cylinder is abnormally low; or the spark plug wires have excessive resistance; or the cylinder head gasket is leaking water into # 2 cylinder. It also could be that the fuel filter is clogged; when that happens, there will always be one cylinder which goes into a lean misfire before the others do. An EGR valve which is not closing completely or a vacuum leak in the intake system could also do it.
Your coolant leak might be in a head gasket, as noted previously; but the leak also could be external; coming from the radiator, water pump or a hose.
To sort out this situation; I would run a compression check on all the cylinders and make sure the pressure readings for each cylinder are written down. Please check these readings against the factory minimum allowable compression pressure standards. If any cylinder pressure is below the minimum; the motor will not be able to be tuned to run without missing. There is also a second standard which must be met; no two cylinder pressures can differ by more than 15%. If either standard is not met; the cause of the failure will have to be repaired.
The location of the coolant leakage can be found when the engine is turned off; by pumping about 15 psi of air pressure into a radiator which is fillled with coolant; keeping the pressure at that level for a full three minutes; and checking for external leaks and for coolant on the # 2 spark plug.
Pressurizing the cooling system to the maximum allowable pressure will cause otherwise undetectable leaks to become very visible.