Stuff being blown up still counts towards production, but it doesn't actually add any value. Changing tanks in storage to tanks blown up on the battle field isn't actually good for the economy, it just looks like it for some metrics.
The US economy (and much of the rest of the world) actually went into recession after ww1. Then after ww2 you have to consider that the US was one of the very few industrialized countries that didn't get its cities blown up. The war was 'good' for the US mostly because it was much worse for everyone else.
Yeah that's tricky, I'd rather accelerate brain drain. But Russia has used immigration as a weapon before, targeting propaganda and sympathies in areas Russians immigrate to.
But Russians are still getting out somehow, conscription is a great motivator.
They aren't, but Russia has used their presence as one. What was one of their justifications for invading Ukraine? The Russian speaking population and historic ties in the easternmost regions of Ukraine. They've also shipped millions of people into Crimea to shift the demographics and public opinion there.
The Russian speaking population weren't just immigrants though, those were people born in Ukraine who spoke Russian as their native language. Kinda different.
Some definitely, but not all. And it's multigenerational too. The USSR moved people around a lot too for demographic control to minimize separatist tendencies.