Giving rings, frequently of precious metals, as a symbol of marriage is, once again, a cultural value and one that dates back thousands of years. De Beers simply increased the popularity of one rock. They didn't even invent the tradition of diamonds which again predates their founding by hundreds of years.
As for the Russia thing, do you think if our tech companies were so directed they could not find evidence of, for instance, Saudi Arabia, Israel, or any other countless other nations also funding various organizations, purchasing ads, etc in ways that they perceive likely to benefit their interests? In my opinion the only reason the Russia thing was given the light of day is because Hillary lost and threw an extremely well connected temper tantrum in an effort to undermine the presidency. And Russia makes a pretty good bogeyman due to the mixture of their 'outsider' status and competence of their agencies and individuals. In any case it's obviously not some meaningful cultural shift. At best they played a negligible role in appealing to preexisting divides within society.
Soft power I think is a much better argument. The Cold War era in particular would be much more meaningful than e.g. Voice of America et al as funded by the USAGM. But there too I'd question the notion of marketing. This [1] is a list of socialist states, past and present. These nations invariably end up resorting to force of arms, suppression of expression, and repressive secret intelligence organizations in order to sustain their power. Unsurprisingly, they don't last long. The USSR, lasting almost 70 years, had quite a long life relative to average. When people have no freedom and your economy collapses (at least relative to free market societies), a nation is already on its way out.
People longing for free markets, free expression, and so on are not doing so because of marketing from the freer nations. The desire for freedom is inherent. It was their governments holding them back. And while we certainly played a significant role in flaunting our successes, the USSR would have inevitably gone the way of all socialist nations. As an aside, no conversation on this topic is complete without this [2]. It's a great list of some Soviet Era jokes told within the USSR. You couldn't criticism the government in public, but private politicization and criticism was ubiquitous.
[1] - https://en.wikipedia.org/wiki/List_of_socialist_states
[2] - https://www.johndclare.net/Russ12_Jokes.htm