2002 Dodge Intrepid engine and immissions issues

About a year ago my car was running rough at idle and the MIL light was on, the roughness wasn't severe but I still noticed a difference. I took it to a mechanic who replaced the evaporative purge valve. It seemed fine for awhile but the problem came back. I went to a different mechanic for that, but also for an unrelated problem concerning my oil pressure light coming on. He informed me that the carbon canister had to be replaced as well as the evaporative purge valve. The diagnostic code showed a medium and minor external emissions leak. He informed me that it wouldn't cause any damage I just wouldn't pass emissions. He focused on the oil pressure problem and replaced my oil pressure sending unit.

Now onto the oil pressure issue since they may be related. The day before I went to this mechanic, I noticed my oil pressure light come on after driving the car for about 20 minutes and then stopping at a light. I was on my way to work and since I was one mile away I decided to just go to work and check it there. The idiot light stopped illuminating when I accelerated. After checking, my oil level was fine but extremely dirty for only having 900 miles on it. I took it in the next day where the mechanic replaced the oil pressure sending unit. This didn't solve the problem and I brought the car back. I was later informed that the engine had loose engine bearings. Since I don't have the financial means to buy another car and would get very little for the trade in, I had to make due. I replaced the oil and put in 20w-50 oil. It has been 5 months and the light has never come on since, my oil dirties at a very slow rate. The last change it had 4000 miles on it but the oil appeared to have the wear and dirtiness of like 1800 miles. The engine doesn't burn oil

I replaced the evaporative purge valve and carbon canister 3 months ago. My car passed emissions and everything was fine until two weeks ago when my MIL came back on it had the same diagnostic codes as before ( yes I learned how to get the codes from my car without going to a mechanic. The car ran a little rough at idle. The MIL has disappeared since and the car runs fine.

Here are my questions could the emissions issue be caused by something like the duty cycle valve which wasn't picked up on the codes and would be an internal leak not an external one? Could this have damaged my engine and also be the cause of the external emissions component failures? It may have built up carbon deposits which affected the bearings. While the idiot light no longer comes on my oil pressure could still be lower than normal, I don't have a gauge on my dash that reads oil pressure, is there a way to install one with out great cost or continually monitor the pressure myself? Could the mechanic have simply made a mistake and it might be my oil pump, or would he likely know the difference. Could it have been simply that the oil was contaminated, it was a 10w-30 but it got very dirty it could have lost its upper level viscosity? I always used 10w-40 before all this happened. Is it a cause for concern if my oil is dirtying at a very slow rate now? The car leaks a little oil from the pan, but doesn't seem to burn oil which isn't consistent with a car with severe wear in the piston and bearing areas. And one final question; I have used Lucas oil stabilizer a few times before all this happened, could this have gummed up something or pushed the bearings further apart?

Thanks your input would be helpful.