You've referred to a few people claiming that the Germans didn't have a Jewish extermination plan in their concentration camps. I recognize a few of the names, I don't have time to look up all of the unfamiliar ones.
The Nazis ran 4 types of detainment camps, lets call them Concentration, Work(forced labor), Extermination, and Transit
As it became clearer that Nazis would not win against the Allies, the Nazis stepped up efforts in the Extermination camps.
As far as I know, the American forces only saw the results of the detainment camps, and never got to the extermination camps (they were in territory that eventually was held by the Soviets).
There's a good book on how the empires of Hitler and the USSR squeezed between them the jews and slavs of Eastern Europe, resulting in millions of deaths, "Bloodlands"
https://www.amazon.com/Bloodlands-Europe-Between-Hitler-Stal...
Also, how about Einsatzgruppen? Are there witnesses claiming they didn't exist either?
edit: Also, what's your response to the Wannsee notes and Posen speeches? Even if hypothetically the gas chambers never did exist, there's clear evidence they very strongly intended to kill all European Jews. Clear intent + 6 million Jews missing + a whole lot of mass graves seems like a pretty cut and dry case, even if you have to resort to saying they were all starved/worked to death and shot instead of gassed.
Incidentally, the exact same pattern follows with the other holocaust denier in this thread.