I have the same problem equating this "existing" moment to the reality of an infinity of futures, yet I still feel as though the "future" moment is already "set", immovable.

My gut feeling is that future moments are "set" so I accept that on faith. I may never understand how "set" and "infinite" futures can both be true; I simply accept it as fact. Future "moments" are already "set" but "set" in water, not stone. Every moment that we pass is set in stone, the turning of the wheel cannot be changed. But what that moment "is" may continually change. Still, and ever moving.