Have you tried some live data to see what pressures the dpf sensor is seeing?
On or around the dpf will be the sensor and it will have two small pipes from the dpf leading into it, one from before the dpf and one after, in a nutshell this sensor measures the pressures before and after and if there is to much of a difference it knows the dpf is blocked, or if there is no difference at all the sensor knows the dpf has been gutted, if this is the sensor you have changed you could have a knackered dpf, but then again you could have some mouse/rat damage to the wiring to said sensor, hence the need for live data.
C3 oil covers pretty much all the bases, C= cat convertor.
20k is way to long between oil changes, I would say 10k max and less if you are going to keep the car.
If when you sort out the dpf, the oil pump casing should be at the very top of your to do list, I cannot stress how important this is.