@Maelstrop
I don't know about all Holocaust deniers, but the one I'd a disagreement with online on Holocaust Memorial Day (when else) firmly believed that It's All Fake. Every last bit of proof of the Holocaust has been faked.
It seems that some of them think Holocaust denial is illegal across the EU, so we're all being forced to "go along with" the accepted narrative & anyone who dares question it will be imprisoned.
He was almost eerily polite though. You feel as if Holocaust-deniers should yell at you/be profane when you keep on pointing out the flaws in what they're saying. But he didn't. I mean, I don't want to be being abused online, but the thought that maybe someone nice & normal-seeming could be thinking this mad shite is... alarming, to say the least.
He did seem, however, a bit thrown by someone who, when told "you have to read Solzhenitsyn!" asked which books he had in mind & then refused to agree that "Two Hundred Years Together" is worth their time, filled as it is with demonstrably false claims & anti-Semitic ramblings.
He really came apart when he started trying to claim that the word Holocaust wasn't used until the 1970s. It's in the founding documents of the state of Israel. Obviously the word itself has been around since the medieval period, but with its original meaning of mass-death-by-fire was unlikely to have been applied to earlier smaller-scale slaughters of Jewish populations.
He also wasn't keen on my pointing out it wasn't only soldiers who liberated the camps (you'd UNRRA teams going in to help, so people like Girl Guide Leaders; Red Cross volunteers; & Quakers) so the idea of creating propaganda in the first place, much less maintaining a conspiracy of silence for all these years, becomes even more absurd.
It was as if he'd read a pamphlet (& maybe a condensed version of that sodding Solzhenitsyn - he'd certainly no idea of the context, nor any of his other works) & was determinedly sticking to what it told him. They are so determined that it didn't happen that they will denounce any & all evidence as fake. They've some mad ideas about the gas chambers not really being gas chambers because the size is wrong (or something) & of course if they admit to there having been crematoria, that's because people die of natural causes & that's the most hygienic way to dispose of a body...
So yes. Short of taking them back in time to see for themselves I don't know what proof they'd accept. And to be honest for lots of them that probably wouldn't be enough. It's a pretty horrifying state of things, really.