This makes me conclude that Russia is just too big. If you break it down into Russia I and Russia II then it becomes a near certainty that the EU will take in both

In this case you'd probably talk about Russia I to X at least, with North Caucasus, Tatarstan, Bashkotorstan (mostly Muslim), Yakutia (partly Yakut), Khanty-Mansi Region (almost no population but huge oil resources, I bet EU would love to have them in), and, last but not least, the Central Russian Orthodox Republic, after accepting which the EU will truly learn what does it mean to have to heavily Christian countries (another is Poland) on different sides of the religious divide...
     Frankly, you don't want THIS experience which will generate endless possibilities for discussing racial and religious reasons to not admit one of these pieces... besides, citizen of today's Russia (Alpha and Omega) would start hating the EU the first time this idea gets aired, and the whole exercise would be moot.