It is possible that the dose was not meant to kill him, just incapacitate for several months and take him out of the running.
Some dioxins are fast-acting.
None that I know of. TCDD and related chemicals act through a genetic mechanism. Within a couple of hours of exposure, you start to see some effects at the cellular level--changes in certain protein levels, up and down regulation of various genes--but it typically takes days or weeks for overt symptoms to appear. Because it's acting through modification of gene expression, and not through direct action on some target (like carbon monoxide acts on the hemoglobin in red blood cells), it does not act quickly.
Weirdest poisoning case I've ever heard of.
Arsenic works slowly too, with more deleterious and dramatic effects.