Will the Balkans be the site of American-Russian competition?
Yes, but I don't think this arena of competition will be a Russian Plan B as America halts a withdrawal from the Middle East and NATO reacts in the Baltic region.
I think Russian efforts in the Balkans--with Greece as the potential big prize (thought I don't think it is likely, it is possible)--in a Russian effort to increase their presence in the eastern Mediterranean as a forward line of defense for the Black Sea.
Although Serbia which may not have gotten over two American interventions against Serbia in the latter part of the 1990s is the immediate platform for Russian effort.
And efforts in the Balkans distract from the main front in Poland.