Did Mendel ever mention Darwin? I've never read that he did. Mendel's work shows that he thought he was working on a different mechanism for evolution. Actually, if you travel to the monastery where Mendel lived, and ask the monks very nicely, they may show you their original copy of Origin, with what are believed to be Mendel's handwritten notes in the margins. In fact, Mendel recognized that his own work supported Darwin, and sent papers to Darwin explaining his own work and how it related to that of Darwin. There is, however, no evidence that Darwin received or read Mendel's letter, or if he did, that it had an impact on him, as he never mentioned it. So, Mendel clearly knew about Darwin, and far from believing he had disproved it, saw instantly how his work reinforced it.
Cool! I'd love to see what he wrote in those margins. Has anyone translated it? I'm not surprised that Mendel supported Darwin. They were working at different levels on the cause of speciation. Mendel was looking into the source of variation while Darwin was looking at the larger picture of the effect of time and environment given variation. All I know of Mendel I've read on MendelWeb. His paper isn't that difficult to read.
As for Darwin knowing about Mendel, I know that there is no evidence that he did. At least those who have his papers have found nothing to that effect. It's too bad too. Unfortunately for Darwin, Mendel's discovery wasn't really recognized until the 20th century.