In early 1990s most of the population was declared atheists, but because atheism was one of the pillars of the rejected Soviet rule it was being reexamined by the people. So there was a lot of orgs of all kinds who flew in to preach and increase their denomination base.

The fact that to the outside Russia looks orthodox Christian today mostly reflects last 20 years of the state efforts to build an official religion, not some deep predisposition of the population. My 2c.

Yes and no, it is definitely easier for the state to build on existing 19 century traditions then to invent a new religion..

I think those 19th century religious traditions were thoroughly gutted by the second world war and 70 years of state suppression.

But a bigger question is why does a secular state is building a state religion?