The politics of losing

I'm totally against king-making just because you "want to see 'player A' win and not 'player B'". Everything you do should be in respect to yourself eventually winning, no matter how slim those odds might be. However, I think the odds are never quite as slim as they seem.

True, big empires can gain a lot of momentum in this game and it can often seem hopeless, but every turn has a good deal of luck involved and real cooperation between allies can be devastating.

So in your scenario, I would say try to affect a balance of power (support #2) until you can build yourself up enough or the two big dogs bleed each other dry.