This "national defense strategy" is just a made up tale, another episode of russian propaganda that keeps poisoning generations after generations.
It's how empires operate. It's not fair, but it is what it is. For an example of how this turned out when the shoe was on the other foot, look at Cuba. The world nearly ended in '63, because the American empire needed a 'buffer colony'.
It's been a lifetime since the revolution, communism as a world ideology has been dead for a generation, but Cuba's still paying for the consequences of flipping allegiances. (The western world, sans the United States has normalized relations with it a very long time ago - so it's not an ongoing ideological problem...)
No. Nations don't throw national resources away, especially in modern times. If they're spending money, their lives, political capital on this they're profiting from it in some way.
Russia has to control the black sea and they have to control territory all the way to the Dneiper river. If Ukraine realigns to Europe and NATO they have to at a minimum have control over that territory to have a defensible position. This has been true since the days of the Russian empire, and this is one of the historical reasons for a Russian ethnic majority east of it.
That’s a descriptive opinion on the matter. Not a normative one.
If Russia did not get an ex-KGB absolute monarch traumatized by the fall of the Berlin wall, the country could've joined NATO by now. Pretending that this particular made up "national defense strategy" is the only one possible, is just caving in to the Putin's propaganda.
Even if Russia for some dumb reason chooses to be perpetually against the West even after the fall of communism, it does not mean that the West automatically becomes interested in invading Russia either. Even in this case the "national defense strategy" is just a cover-up for the unprovoked aggression.
Russia joining NATO in good faith would change it for sure but would not make it obsolete.
Who would that alliance be (ostensibly) protecting themselves from? China? Taking that kind of stance (EDIT: against China) would not be in Russia’s interest.
It's not a national defense strategy, it is a national security one. They need to control the paths for gas exports to Europe to maintain Europe's reliance on them for heat in the winter. It makes them powerful.
If you look at national strategies for nations and territories, even beyond the time the current state has ruled, for every major or minor power throughout history, you'll find similar strategies because the underlying realities remain the same.
You don't need military action to provoke. The West has been trying to incite regime change all over the region, Ukraine is a prime example.