They are communists and they support the SMO. I know there are a lot of problems with them but frankly those two things are the most important red lines from my point of view.
Too many of the other self-proclaimed communist, socialist and anarchist groups in Russia are pathologically liberal, way too close to western leftist views on Russia, and more obsessed with being against everything their own government does than they care about fighting imperialists and literal Nazis.
Yes, there needs to be opposition to the very reactionary domestic policies and rhetoric of the Russian government, but not at the loss of critical support for their anti-imperialist and anti-fascist actions.
The KPRF are far from perfect and they will need to have a lot of internal ideological struggles to reckon with their own culturally reactionary tendencies (as will many anti-imperialist forces around the world which we nonetheless critically support), but they are the only ones who are sufficiently embedded in Russian society, institutionally, socially and historically, to constitute a real communist opposition to the bourgeois-nationalist ruling party.
The difficult balancing act for Russian communists is to simultaneously remain a distinct opposition to the current bourgeois government, advocating for a new socialist organization of the economy and society (possibly taking inspiration from China's successful model), while at the same time not becoming an unwitting tool of imperialism for destroying and subjugating Russia, as many ultra-leftists invariably end up doing whether they intend to or not.
Because Russia is too important for the global anti-imperialist struggle to be allowed to suffer the same fate as Yugoslavia, which is what the imperialist West wants more than anything else since they now know they cannot subjugate and neo-colonize Russia through compradors like they did the other former socialist states... Russia is just too big for that.
And if they managed to succeed in installing some kind of Navalny-like puppet via a color revolution, that would still only be the first step toward the total dissolution of Russia, which is really their ultimate goal. They would use such a puppet to stir up ethnic conflict and inflame ethno-nationalists (including Russian ones) who would seek to dissolve the multi-ethnic Russian federation.
(Incidentally, this is also the model they ultimately have in mind for China, hence all their support for separatism and ethno-religious nationalism in China.)
Conclusions:
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Russian communists need to recognize this very serious threat and work against it; preserving Russia's unity and sovereignty is paramount; I believe the KPRF understand this.
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They need to recognize the primary contradiction in the world today - imperialism and western neo-colonial hegemonism - and act accordingly; I believe they understand this also.
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They should not fall prey to idealism and dogmatism, but instead adopt a pragmatic strategy when it comes to dealing with the current bourgeois government while continuing to push for socialism; again, for the most part they get this right, though they could be more self-confident and more active on the last part rather than resting on the laurels of their predecessors.
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And they should make much more of an effort to combat right-deviationism and tailism on cultural issues. This is where their main weakness lies today and what is preventing them from having a proper mass line.
I know this is not exactly what you asked for and i'm sure someone can recommend some reading materials to you, but this is my view on the situation with the KPRF today from my limited knowledge as an outside observer.