Is “Russias” the plural of “Russia”, in the sense that this is how they relate grammatically?
The reason that I suspect that they are not plural-singular is the following example.
 I see the egg.
 I see the eggs.
 I see an egg.
 I see egg.
[a] I see the Russia.
[b] I see the Russias.
[c] I see a Russia.
[d] I see Russia.
It seems to me that  and [d] differ semantically.
“Russias” is certainly plural, but I think you’re right about there being a possible objection to calling it “the” plural of “Russia” because “Russia” is a proper noun (and therefore semantically definite, even though it “anarthrous” or not used with a definite article). You can’t “pluralize” a proper noun without somehow converting the meaning: for example, you could be talking about different things named Russia, or hypothetical different versions of Russia, or the different ways that different people experience or perceive the country of Russia. So as you have hinted, the singular counterpart of “Russias” could be considered to be the countable indefinite “a Russia” (rather than “Russia”).
The situation is the same for any other proper noun that is typically used as a singular noun (Finland, Australia, etc., or personal names like “Alice” or “Andrew”). You can use a plural form, but the exact “singular counterpart” of a noun phrase like “Andrews” could be considered to be “an Andrew” rather than “Andrew” by itself.
We see a comparable phenomenon of semantic “coercion” with the plural forms of words that are usually non-count nouns, like “wools” or “gravels”. The meaning changes (typically to something like “types of X”) because the usual meaning of a word like “wool” or “gravel” is not compatible with plural semantics.
In the end, though, the concept of “the” grammatical plural of something may not be very useful. It might be better to just think in terms of plural forms and singular forms, definite forms and indefinite forms.
E.g. “the egg” is a definite singular noun phrase. “The eggs” is a definite plural noun phrase. It’s not really clear that “the eggs” is “the” plural of “the egg”–the definite plural noun phrase doesn’t refer to multiple instances of some one particular egg, it refers to a definite set of multiple eggs.