If I build their slicer, not modifying any line of code, then accessed using that binary, would that be acceptable? If not, why not, considering it is identical to what is on their website?

If I made any changes prior to building, would it still be acceptable? And if not, where is the line? What is the legal basis, any precedent? How much of the code may I modify before I cross an invisible threshold and somehow "bypass" an "authentication" (neither fit UA anyways, either for law or other purposes unless one can provide any evidence that it ever has).