What are the key elements needed to prove Concept after getting a provisional patent filing accepted in order to complete a final filing documents?
You do not need to prove the concept by documenting experiments unless it involves biology or possibly chemistry. You do to have the solution in hand, requiring no further invention. But it can all be on paper.
A patent application, to have any real value, needs to show that you are "in possession of the invention", and needs to "teach someone skilled in the art how to make and use the claimed invention without undue experimentation". It does not need to show a particularly efficient or production-ready implementation. You can't hold back critical details of whatever you think is the best way to do it. The patent application does not need to be down to the detail of manufacturing drawings as long as you are not hiding the "secret sauce".
Although a provisional application does not need the same formality of a non-provisional application, it does need to contain this same level of content or it might not do you as much good as you think it does if you end up needing to rely on it to establish a priority date.