If Sweden allies Poland then Muscovy is dead. The problem is that when Poland forms PLC and gets 3 ideagroups locked, it will have much better army quality (+10% inf CA, +48-53 Cav CA, +15% Morale, +5% Discipline) and much more development. Who would prevent Poland from taking back Southern Baltic and Russian lands back?
Muscovy is balanced in MP by not placing any players on hordes and by weak start of PLC. Protecting Novgorod as Poland works only against inexperienced players ("Subjugate Novgorod" mission is taken from day 0 while Poland rests in interregnum and improves relations).
Well, the most decisive victories Muscovy had against PLC were in 17th century (at this time, by logic of the game, PLC will have Space-Marines-in-training). The draw in Livonian War had its own reasons (e.g. newly formed PLC, Crimean raids, minor Sweden intervention, problem with inner circle of nobles in Russia).
Also - your question is strange - Russia should be able to do that. Among others things, player on Russia should be really good diplomat (because Poland player can offer to share Russian territories (with, maybe, backstab in mind), while what can Muscovy player offer? Alliance?), good militarist (having quantity in ideas helps somewhat, but some aspects of quantity group are better, and game mechanics atm doesn't support levee en masse war style and, heck, just doesn't support using manpower overall) and good strategist (cause he should think, like, 5 steps ahead).
We can compare Muscovy+vassals to Poland+vassals+union.
Poland can have 25k army, with vassals giving another 14k and Lit giving another 27k. Union will fire by 1450 100%.
Muscovy can have 29k army, with vassals giving another 14k.
So, basically, Poland has 24k more from union. Because game mechanics doesn't represent how the union worked (Russian managed to conquer Smolensk when Lithuania was under union, because Polish king was afraid of Ottomans).
I'm not sure how this is balanced. Considering, that during hard war between players AI can attack Muscovy player. And the problem is that this horde will take land, while, mainly, in history during this period hordes just raided.
I was able, in one game, to deal with Novgorod and Lithuania (there was player on it because I decided to play Muscovy). The victory was hard and, well, devastating. I'm not sure, that I would be able to win, it this was full union.
I agree that, maybe, it is balanced. But, in some huge leagues, there will be players on one (or even more) of three hordes.
Still, I think, that from all major countries Russia has one of the worst combination of ideas and position.
Funniest thing, that PDX are railroading Prussia, while appearance of this country was one hell of a luck. Muscovy was railroaded by its previous rulers.