If achieving “nirvana” means that we will be liberated from the illusion of consciousness, how then will we know (be conscious of) our achieving this “nirvana”? How can we be “conscious” of this, if “consciousness” will no longer exist?
At the point of Nirvana consciousness ceases momentarily and starts again when you come out of the Phala Samapatti. At this point you see the the collapse and reforming of the links of Conditional Relations / Dependent Origination.
how then will we know (be conscious of) our achieving this “nirvana”?
You have seen the cessation of the 5 aggregates and start of it through full experiential knowledge of seeing the working of cause and effect.
How can we be “conscious” of this, if “consciousness” will no longer exist?
You need not be conscious about Nirvana but you insight into cause and effect will make a paradigm shift in your outlook about the 5 aggregates.
Also see: How does stream entry occur?
Also following quote might be of interest. This is slightly explained in more detail during the 20 day course.
What do you think is the ultimate goal of Vipassana?
SNG - The ultimate aim is to clean the mind. Nirvana is beyond mind and matter. It can come to you anytime depending on how pure you are. In that state, all your senses stop functioning. It could be for a second, a minute, or longer, but for that brief period, you are beyond all sensation, all thought.
-S N Goenka, from interview published in the 'life positive' magazine.
Nirvana is the end of greed, hatred & delusion. Consciousness is not an "illusion". What is an "illusion" is delusion. When Nirvana is attained, the mind remains consciousness. You can read here:
In certain sects of buddhism, there is the theory that there are ten worlds ranging from hell to buddhahood. Each of these "worlds" however, is really a life state, or living condition that exists inside of one's self. The tenth world, buddhahood, is also called nirvana. If one is in this state, one is without delusion, but still conscious and able to go about daily life.
Additionally, not all sects of buddhism's goal is to "free one's self from the delusion of consciousness", but to instead be truly happy and help others. In this sect, there is another "theory" called the ten factors. They are what the true entity of all phenomena consist of, and, if correctly using the ten factors, one is freed from delusion. If you want to learn more about the ten factors, this website gives a much better description than I am able to: The Ten Factors
I hope this answers you question well!