The burden of proof is on you to explain how it DOES work.
No it isn't. I'm not trying to prove or disprove them and never claimed otherwise. If you want to claim that they cannot work then you need to provide a testable theorem to back that up. If you want to claim that they can work same thing applies. If you are merely trying to refute claims that someone has developed a quantum computer when they haven't then you merely need to clarify your position.
Here is what I think we know right now. Some scientists apparently have created functional quantum computers with small numbers of qubits in labs. These lack sufficient qubits to be generally useful but do appear to indicate that useful quantum computers are likely to be possible. If there is a quantum computer with enough qubits to be generally useful I am not aware of it and there is no public indication of any breakthrough at this time. There appear to be substantial technical and theoretical problems to be worked out before quantum computers become a reality.
Is it possible that someone, somewhere has actually constructed a useful QC in defiance of all the skeptics
As far as I know they are all in the proof of concept stage in physics laboratories with very modest numbers of qubits. Never claimed otherwise. It does not follow however that quantum computers are an impossibility. Based on my understanding of the work accomplished thus far I suspect they probably can become a reality eventually but I make no claims regarding when that may be.