There could be other guarantees beside NATO membership.
Reality of the situation on the ground is without direct intervention by a major power, Russia is going to continue to gain more and more ground, with or without US support. At a snails pace yes, but my bet would be Ukraine runs out of resources (men, material, etc.) before Russia.
I'm thinking that the other goal of negotiations might be to show that Russia's demans are unreasonable
Which could, ironically pave the way for increased aid or even direct intervention