See Dwight Wilson's book Armageddon Now! for documentation.
Hardly. He's basing them on the opinion of a single reporter. He provides no facts, no quotes from prominent Dispensationalists before and during WWII, nothing to substantiate his point. He literally does not provide enough of an argument to respond to.
If I posted a counter-article in which I posted a single sentence from Lindsey's The Road to Holocaust as the sole evidence to "prove" that Replacement Theology was what was to blame for the deaths of six million Jews, I daresay you'd find it less convincing than DeMar doing the exact same thing here. I also daresay that you'd be just as quick to accuse Lindsey and myself of slander as I am to accuse DeMar.
DeMar has falsely represented--lied--about what those in another theological camp believe. There were any number of legitimate routes he could have taken to criticize Dispensationalism on theological grounds (and I have in fact seen him do so in the past); I wonder then why he's going all-out with the slander tactic lately?