I think if you want to get as close as you can to O Sensei's aikido you have to train in the Iwama tradition. M. Saito was with O'Sensei for 23 years and trained with him outside of standard class times. I know this will anger a lot of people but that is just the way it is. Whether or not that is good or bad, I do not know. Personally I have a strong background in Iwama style but I also do not think that I can't learn from other styles of aikido and even koryu.
I always found it interesting when these shihan would claim to be uchi deshi for O'Sensei after the war and did not spend any time in Iwama.
If any aikido style today had the "real goods" or was operating at a level closer to the Founder, people would be flocking to it, just like people flocked to Takeda and Ueshiba looking for something especial. Such skills would be evident in the students of this style and people would want it. The fact that there's no mass migration to any particular aikido style, and so many styles and organizations survive in parallel and none can claim superiority based on skill, tells you that everybody is pretty much on the same relative level compared to the aikido propounded by the Founder, regardless of whether your lineage is pre-War or post-War, or comes from Shingu, Iwama, Tokyo, etc. As Marc Abrams suggested, it's less imperative to discuss lineages thatn it is to concentrate on what the Founder was really doing and work to replicate it.