The facts about free will, and Krsna's promise that once returning to Vaikuntha, one never again falls down to the material worlds?

Over an eternity, not even Krsna's promise to the jiva-souls, saying they will never again fall down to the material creation, can interfere with the jiva-soul's free will to again fall down if they choose to do so. 

If Krsna forced His will on the jiva-souls, then that puts an end to free will. Kṛṣṇa's promise that one will never again fall down to the material creation, only becomes fact IF the jiva-souls agree with with Him.

In Bhagavad Gita As It Is, it is explained Krsna left the decision for Arjuna to choose. 

It implies that in the Vaikuntha planets and Goloka-Vrindavana, Kṛṣṇa does not impose His will on anyone. 

Krsna has granted free will in the Spiritual worlds so the jiva-souls can always have a choice on how to voluntarily contribute and serve Krsna, including even rejecting Krsna if they choose to do so.^*^.