> night doesn't become day in a single instant.
How about 75 years?
> shows that you obviously grok the gravity of the situation.
The situation is thus: humans are one mean species, capable of great deeds of kindness as well as atrociously horrible deeds of evil. Throughout history people have done both, places where horrible deeds were done - say during the rise of the Mogul empire in India or the Turkish genocides (yes, plural, the Armenian genocide was only the last in a row stretching over a period of almost 40 years) on Christians in the Ottoman empire, the atrocities of the GULAG system in the Soviet Union and probably even during the cultural revolution in Mao's China - also showed people risking their lives to save those who were being persecuted, usually putting those who tried to save people in dire risk themselves.
Many people have learned to live with the knowledge that back in time ranging from centuries to a few decades their ancestors were subjected to annihilation, torture, murder, eradication, extermination, marginalisation... the list goes on. Those people have not forgotten but they generally realise that those who committed the atrocities are long gone and that the currently living generation of their ancestors' persecutors are not to blame for their ancestor's crimes. Those who do not realise this end up in generation-spanning blood feuds and vendettas. Wars were started over such things, leading to even more atrocities, sowing the seeds for yet more wars.
So yes, you can buy a Volkswagen without feeling the need to call it a "Nazi car" just like you can buy a Mitsubishi without calling it a "Jap car", an IBM computer without labelling it a "racist computer", eat a south-African "Outspan" orange without referring to it as an "apartheid orange". You may want to leave that iPhone on the shelves to signal your discontent with the circumstances in which it was produced because that is a problem which is happening right now instead of centuries ago. It is, in other words, something which can be changed in the here and the now. The same goes for that example of conflict diamonds you gave, nobody should buy those or any product they are part of.
Calling Volkswagen a "Nazi car" does not achieve any purpose other than signalling to others that you consider yourself to be virtuous, "not a Nazi", in your own words expecting everyone to give you kudos for it. Most people consider themselves not to be Nazis without needing to call out Volkswagen, Agfa (does that still exist? It was a spin-off from IG-Farben), Mercedes or Thyssen-Krupp. That does not mean those people are not aware of the history of those companies, it just means they can separate the past from the present. It does not mean they condone the crimes committed by those companies, it just means they realise that Volkswagen in 2020 is run by different people, with different goals, different outlooks on life, staffed by different people than it was back when Hitler and Porsche came with the first "Volkswagen" (people's car).