There are lots of reasons you shouldn't put duracool in your car, so its worth getting the 609 certification. It only costs around $19. If you have confidence in your repairs, once you fill it with r-12 you should not have to touch the system, or "vent" anything for years to come. Airconditioning systems are designed in relation to the refrigerant that will be used, hence they don't work well with non-factory refrigerants. You already discovered this with the r-134a, so it seems odd, after all of your hard work, that you want to put another sub-standard refrigerant in the system. On a lighter note, you indeed can put oil in the receiver. I distribute oil throughout the system when I have it open, and putting some in the receiver/drier is just fine.
'84 300D, 176k
'90 300CE, 166k