Oh but it is, Sidious is giving more and more power to the military in the name of 'security' - at this point the Republic is half way to becoming a military state, the military have more power than the Jedi, and they can do what they want. And by this stage Tarkin is becoming Sidious' personal protege, Sidious made many of his 'friends' who would later become the leaders of the Empire, during the Republic era - Tarkin, who is now an Admiral, is the chief of these. He and the Chancellor and likely working together.
But of course they'll show evidence, it will just be faked. The Jedi don't have enough power to challenge the legitimacy of it. Although its clear that they're not completely convinced.
Not yet, the Senate was still there and Palpatine was still the Chancellor rather than the Emperor. Even in the Empire era, he's not some personal protege. Tarkin was below the 13 Grand Admirals.
If they could show fake evidence Tarkin would not say "the sound doesn't work".