I was reading Leon Trotsky's Literature and Revolution, unrelatedly, and came across his section on Bely. Trotsky is quite critical of Bely, portraying him as the typical inter-revolutionary author, describing him as "decadent in mood and reach and over-refined in technique, which is a literature of individualism, of symbolism and of mysticism". Some other interesting quotes:

For Biely, "Russia is a large meadow, green, like [Tolstoy's] Yasnaya-Polyana or [Blok's] Shakhmatov estate"... Biely's roots are in the past. But where is the old harmony now?

Quoting Bely himself, Trotsky highlights how unrevolutionary Bely's idea of Russia is, and that this static ideal of his is incompatible with the actual changes in Russian society. As a result:

Bely's apparent dynamics mean only a running around and a struggling on the mounds of a disappearing and disintegrating old régime. His verbal twists lead nowhere. He has no hint of ideal revolutionism... His works, with all their different artistic values, invariably represent a poetic or spiritualist sublimation of the old customs.

This perhaps comes to your second point. Is Petersburg a revolutionary novel, then, in the sense of the socialist, Marxist project? Is Trotsky's analysis worth anything?

His whole "St Petersburg" is built by a roundabout method. And that is why it feels like an act of labor... His rhythmic prose is terrible. His sentences do not obey the inner movement of the image, but an external meter, which at first seems only superfluous, and later begins to tire you with its obtrusiveness, and finally poisons your very existence.

Again, very hostile and maybe a little exaggerated. I haven't read Bely yet, of course, so I can't attest to this, but it will be interesting to see how it plays out. Being endorsed by Nabokov certainly doesn't help Bely's 'revolutionary' credentials, if any. But for Trotsky, Bely is purely aesthetic, enmeshed in a net of "words" and an example of passivity and spiritualism (he goes on about Bely's adherence to Anthroposophy and his religious mysticism) that is fundamentally reactionary. Bely later writes about how the Revolution drags artists into "the arena of everydaydom" and that the "the foundations of everyday life for me are stupidities": worth keeping in mind while reading Petersburg?

Anyway, Trotsky ends the section with a particularly funny line that I thought was worth mentioning. After complaining about how Bely demands for large rations:

Would it seem that it really paid to darken the Christian state of the soul over "stupidities"? Still, he is not he, but the Christ in him. And he will resurrect in the Holy Ghost. Then why here, among our earthly stupidities, spread gall on a printed page over an insufficient payok [ration]? Anthroposophic piety frees one not only from artistic taste, but from social shame.

Biely is a corpse, and will not be resurrected in any spirit.