Bad crankshaft, seal, or sensor

My car has the same symptoms your list has for a bad crankshaft seal, sensor, and the crankshaft itself. Last month had the main bearings replaced along with rods and other parts that came in the rebuild kit I had. Owned going on four years and it was not cared for at all. First oil change was literally like molasses, in color and thickness. From that oil change until a month ago knocking went from not noticeable by untrained ear to the first sound heard upon starting the car. Surges once it has reached normal operating temperature when put into park or neutral- doesn't surge in drive or reverse though. Replaced baro sensor as it was the only code shown when reading for any failed components. Last month replaced main bearings, rods, etc, that came with rebuild kit. Does have an oil leak up front-has always had dark oil even right after a change and a drive. Thought knocking would stop after changing bearings and rods and etc, but still knocks just as loud and hard as before the four day rebuild. All nuts, bolts, etc, rusted and seized on the car. A crankshaft is hard to find in my area and quite pricey, especially if it may just be the seal or sensor. Mechanic told me it needs new crankshaft. Period. Second opinion says sensor. How do you diagnose a difference between the three? Obviously a sensor or seal would be great compared to actually needing a new crankshaft in general. Can't afford to let it sit in a shop, not many in my area anyway. Don't want to pay for something way more serious than the problem actually is, like crankshaft vs seal or sensor. Any thoughts or ideas are sincerely and tremendously appreciated.