2) What's the progressive/conservative proportions in the United States? If progressives are the majority in the United States, could the US branch of the UMC walk out as one group?
Not easily. The way it's organized, there is no US branch per se. They have five regional conferences in the US and then several conferences covering various parts of the rest of the world.
That makes it even easier then, no? Presumably the progressives are concentrated in a couple regions. Why couldn't they walk out as a group?
Because their church properties would still be owned by the UMC they are leaving. As I've pointed out before, the UMC is not a congregationally organized church. For the most part, the churches are owned by the UMC, not the individual congregations nor by the regions. It's a lot easier for individual members to leave the UMC than for whole congregations or groups of congregations.
Larry Goodpaster is the most prominent Methodist Progressive I know of.
For the record, Wesleyanism is broadly popular throughout progressive and conservative circles. The church won’t schism over social issues, as it has broad agreement on theology.