It would have made their lives so much easier if Dumbledore had taken Harry’s memory of the event, for one reason: they could have taken Fudge into the Pensieve to view the events for himself. After all, modifications to memories are slightly obvious if A Sluggish Memory is anything to judge by, so this would have provided the greatest evidence to Fudge that Voldemort was indeed back. Now, whether or not they could actually convince Fudge to view the memory is another matter entirely (Something along the lines of “I won’t let you waste any more of my time with this useless nonsense”).

So, another reason to add to your list could be that, as Harry’s word is good enough for Dumbledore, and he could watch the thoughts in Harry’s mind while he recounted it, Dumbledore may have simply believed that taking the memory from Harry would be a wasted effort anyway.

I don’t think Fudge knew about the Dark Mark at all. I think, at this point, only Dumbledore, Snape, possibly McGonagall, and the Trio, and obviously Voldemort and Death Eaters knew about the Mark. It was kept a secret within Voldemort’s ranks all through the first war, and Dumbledore learned of it because of Snape turning to the Light. My thoughts are that the Dark Mark faded away the night Voldemort lost his body and powers, and that’s when Death Eaters started to be arrested and turned in to Azakaban. Therefore, no one would know about it. And even with the Order, we all know Dumbledore likes to keep everything close to the chest. I think the only ones he trusts implicitly are Harry (and in turn, Ron and Hermione) and Snape. They’re the only ones he told everything to, including Horcruxes. And Snape is the only one he told about his plan for Harry. I think we’re all dead set on wanting to believe Fudge is so ignorant and short-sighted, that it’s all his fault, where some of it isn’t. I think we have to remember that Fudge is a product of how he grew up (blood purity, half-breeds, and all of those wizarding prejudices), and that’s what makes him so terrible. He grew up with ignorance, so that’s how he lives.

I like all of John’s reasons for why Dumbledore didn’t ask for a memory, as they make total sense, but I also am quite partial to Paula’s added reason. He wanted Harry to have the feeling of catharsis that reliving it all had for him. As we read, it was like Harry was extracting a venom (or did he say poison) from himself as he told the story. And Dumbledore, having lived through tragedy, would understand that very well.

**SPOILERS**
I never really quite understood Dumbledore’s look of triumph. Maybe it’s because I haven’t read DH as many times as the other books, so I think that understanding hasn’t been driven into my brain as fact yet. Is it because Dumbledore knows of the new protection Voldemort created by taking Harry’s blood? I vaguely remember this, but not word-for-word like I do the earlier books.