1) He's not so much a person, as a title, which is this enormous friggin' secret. The real Dread Archmage Diego died 37 years ago, and his apprentice took over. It's been passed down for decades, and there have been at least six different 'Dread Archmage Diegos.' Why do you think he always has that flowing robe and that creepy skull-mask? Heck, one of them was even his grand-daughter! Anyway, the current 'Dread Archmage Diego' isn't exactly an Archmage. Actually, he's kind of in-training, after the last Dread Archmage Diego mis-drew a summoning diagram.
2) Helminster's got this really great connection with this order of Bards who call themselves Those Who Trumpet, and they *may* have been a *teensy* bit overzealous in their promotion of his talents. I mean, it's great to have all these songs sung about your historic deeds, and how your trusty manservant Wong is actually the deposed prince of Hethyr and your girlfriend is the ruler of Haglarond, but it turns out that the immortal Chosen Helminster is actually an 8th level Diviner with a really cool collection of maps and doesn't so much sleep with the goddess of magic, and, indeed, is kind of suffering under a curse from said goddess of magic because of that particular rumor. I guess she's not as easy as the 'Lay of Helminster' suggested.
3) Oh, Frizzt the legendary dark elven hunter with his bi-curious relationship with that magic anime catgirl and his twin swords Twinkle and Moonbeam! Yeah, he's, well, really good at killing Orcs. They're his favored enemy, after all. Dragons, demons, lich-kings? Not so much. He's kinda specialized in Orcs. Do call if you need any Orcs killed 'though!
4) Lemminkainen? No? Oh, Mordenkainen! Oh gosh, he's the one who lives in that tower with no doors or windows, right? Yeah, I heard that Stumpy Sam the Cart-man tried to break into that place once. They just called him 'Sam,' back then. No, I have no idea how to contact him.
5) The Witch-Queen of the Shining Woods? Uh, no. Seriously. She eats people. I'm not even kidding. We leave her alone, she doesn't leave the Shining Woods with her Wyld Hunt Werewolves and eat us. It's an arrangement that has worked well for centuries, and, as Mayor of Town Too Close to the Shining Woods, I will mobilize a pitchfork-and-torch-wielding-angry-mob to stop you if you even *think* of stirring up that hornet's nest! Oh yeah, I've heard the propaganda. 'The Shining Woods is a democratic society, where happy creatively fulfilled elves and humans live together in joyous harmony, singing songs and turning out fine woodcrafts.' Go ahead and think that if it helps you sleep at night. Do you really think they'd be able to undercut the lumber market if those 'happy elves and humans' were getting *paid* to work themselves to death? It's not Santa's Workshop up there...
6) Yeah, the Diagram of Eight. Powerful mages, archmages even, they say. I can't remember, either Larry the Traitor killed them all, or maybe it was that Liche-God Voldemort or whatever, when he tried to steal the power of the gods. Some of them are said to still be alive, or have come back from the dead. Either way, they're not answering the door and my theory is that they've died enough times in the last five years that they aren't taking visitors for the next decade or so. Thank you, come again!
7) Hey! That *is* a thought. I'll call Sheila the Spellfire-girl! She can blow undead dragons out of the sky! Thanks for the idea! You can go now. I wouldn't want you kids to get hurt...
8) All I have to do is snap this stick he gave me years ago, and Thornwylde, the Grand Druid, will be called to help me, to honor our ancient pact. <Snap>
"Oh dear. You, you're not Thornwylde..."
"Actually, I am. I converted to evil and became a lich over a decade ago."
"I didn't know Druids could become liches..."
[ticking off on fingers] "Humanoid. Spellcaster. Must have Craft Wondrous Item feat. RTFM, noob."
"Well crud."