The paradox of the Bodhisattva Path (Bodhisattvayāna) Everybody is born under his own Karma. Indeed, while respecting the Dharma, we need to follow our own paths. Monks live an undisturbed life inside closed communities apart from samsaric suffering. And when they are ready, they try to enlight others under the Bodhisattva_vow
If Sunyata is the ultimate reality, why hasn’t anyone achieved the ability to save others nowadays?
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How (and with all respect) the Dalai Lama and other monks can have reached the Enlightenment if he wasn't able to free in this life ALL human beings?
They are not reborn KNOWING TO BE a Bodhisattva. They can maybe get near it with a hard work and faith, and so they cannot free others from bonds in this life. Furthermore, they can teach a path, which is of course important, but the basic logic of BEING a Bodhisattva contradicts this flow.
How can Bodhisattvas enlighten others if they themselves are not fully enlightened? If they are enlightened, then they would not have been reborn in the first place. If they were reborn, then it means they are not fully enlightened and therefore still working towards it and at this time, cannot liberate others because they themselves are not liberated. And they cannot be liberated until ALL beings are liberated, due to the Bodhisattva Vow. That's the paradox of the Bodhisattva path.
How can the paradox of the Bodhisattva path be resolved?
There is no paradox.  Rather, it seems there may be some mistaken assumptions behind your question.
First, if someone is enlightened that does not mean that they will enlighten all people. For example, The Buddha was enlightened and yet he did not enlighten all people.  In fact, after his enlightenment, he actually wondered whether he should teach what he learned to others!
Second, no one can enlighten anyone, they can only help or hinder people on their quest to enlightenment.  Whether someone gets enlightened is up to that person.
Third, you don't need to be enlightened to help others on their quest for enlightenment.  For example, I am not enlightened, but I can still explain the benefits of mindfulness to another person.  This can encourage the other person to try mindfulness practices, and this can set them on their journey to enlightenment.
Finally, a Bodhisattva chooses to forego Nirvana in order to aid all sentient beings.  Depending on which definition you go by, this aid can be mundane -- such as feeding the hungry, reducing pain, etc... Or it can be sublime, such as aiding people on their quest for enlightenment. Either way, there is no paradox here; the Bodhisattva chooses to be reborn.