The 3000rpm skip in neutral and park is the rev limiter. If it has never done it before I cannot explain why but your Lincoln does have a rev limiter set at 3000 to protect the engine.
The cat effeciency code is set when the front and rear O2 sensors read too closely together telling the PCM that the cats are not performing their job of reducing emissions due to eithr a clogged cat or a cat that is hollowed out. Basically the rear O2 sensors are tattle tales. Most likely this is a cat starting to plug up on bank 2 which corrosponds to the drivers side cat. A plugged cat could be the result of your occasional overtemp on the display due to a plugged cat raises engine temps. Be sure to check coolant level though. If coolant is down a little in the radiator it can create small air pockets whenever the thermostat opens.
Before replacing any converter, have it checked. This can easily be done by a shop with the use of a back pressure guage and checking pressure before and after the cat. Aftermarket cats can be used so long as they are OBD II compliant. If you are not noticing a performance drop there could be contamination on the rear O2. Another reason to have the cat checked first. If it is blocking it will rob power. Misfires can cause the code also but you said it isn't missing a beat which is why I eliminated that.
As for which side is bank 1 or 2. Bank 1 is the side of the engine with cylinder 1. Bank 2 is the opposite side. Cylinder 1 is on the passengers side of your LS leaving 2 on drivers side.
Sorry for any confusion.