Here's my speculation - Sulman was living in Sweetwater in 1162 AS and convinced the Devils to spare him (it's not unheard of. In Heroes III: "Olema spent her entire life in Eeofol, but when the Kreegans arrived, she talked them into letting her serve as a guide to the land, and has since proven her ability to lead.") He then ran off to Ironfist looking ragged, swore blindly he was the only survivor of the Devil attacks, and thus - as an Air Wizard and a man who'd seen the Devils - was a very viable asset to the Court, who are totally unaware of his revised loyalties.

As to why exactly Sulman would be unhappy, consider that he possibly wasn't part of any office. There's an interesting piece of 3DO lore preserved here. It seems to feature Sulman himself on a return journey from Antagarich to Enroth. If you're going to ask just what he was doing in Antagarich, it's clear: Xenofex and his Devils had transported Roland and Tanir from Paradise Valley to Colony Zod, also stopping by in order to make terms for their a deal with Sandro to invade Erathia and kill King Gryphonheart (this is what Xenofex means when he says "my minions have engineered the recall of Catherine to Erathia"), and Sulman went along with them for whatever reasons before being sent back to New Sorpigal. Notice that this lore mentions the "traveller", probably indeed Sulman, comes from Silver Cove, not Ironfist.

http://thespinwardtales.wikispaces.com/Xanthor's+Tales+-+Disturbing+Letter?f=print

Silver Cove is indeed part of the kingdom of Enroth (or at least as of Heroes II's ending), but its leaders were never known for their obedience to the law, so it probably wasn't drilled into Sulman that Morglin and Roland Ironfist are the only indisputable masters of the continent. Now, as you can see in MM6, there aren't many Archmagi in Ironfist, they're scattered all over the continent in numerous cities. Newton is the current High Sorcerer so only he would really have a comprehensive idea of all his underlings' loyalties or personalities, and his memory was blown from too much spellcasting (though Roland was in the Wizards' Guild for a while and might've known a few names).

It's correct that Ironfist has a hierarchy with a few positions in spellcasting, but not many. Court Wizard - only Twillen and Tanir have ever held that position. Aside from that, High Sorcerer is the only other "office" for sorcerers confirmed to exist in the Kingdom itself, the rest are just positions or professions with no monetary benefits that we know of. Considering that some of the Archmagi are living in shacks in Freehaven or the Mire of the Damned, it's not likely that Sulman was overpaid, if at all. Aside from his mentors, again probably the ever-amnesiac Newton, he has no obvious masters before Zenofex.

For his journey west, Roland needed to pass through Kriegspire to reach the Sentinels and required a potent Air Mage to fend off Dragons, flying Kreegans, snowstorms etc. Tanir was an Earth Grandmaster (turned Archibald to stone) but nothing more, from what we know. Obviously Sulman conjured the snowstorm that cost Roland the siege there. I don't know why Roland trusted Sulman so explicitly. He was ridiculously naive in Heroes II and probably hadn't yet let go of that trait, plus - he's a King, more than enough duties to worry about. Regent Humphrey was suspicious from the start of some deceptive intrigue in the Court, but was probably focusing his efforts in the "wrong" direction (Lord Haart). But that doesn't explain just why Roland poured out his trust. Maybe Sulman participated in the Succession Wars against Archibald - he's not mentioned in H2 but there are several nameless Wizards and Archmagi mentioned to assist him. I don't really know, but if anyone has contact details for Greg Fulton (storywriter from MM6 onwards), please send 'em here

Potbello is a different matter - nothing to suggest it but I guess we can envisage the possibility that he guessed the adventurers weren't actually Sulman - no Baa cloaks, nothing wizardly about them. That's if he knew who he was supposed to be meeting, of course, but we can't really be sure - if he did suspect the adventurers then maybe he gave them that money just to move them along as quickly as possible - note that the letter in the Supreme Temple shows the Kreegans are already aware of the adventurers, probably because of their actions but maybe also because Potbello told Xenofex all about them and how they weren't Sulman. 1000 gold is a crappy amount of money, but still I don't think money was Sulman's primary motive in betraying the Ironfists in the first place. Getting back to the question of what Xenofex offered his followers, there's several possibilities. Silvertongue has almost definitely been promised Roland's throne ("soon I will be your new king!"). Druthers and the Moon-worshippers could simply be out for power, I don't think the Celestial Order was trying to spin a vast web of deceit in corrupting the Monolith. Sulman? Read over The Letter's last words - "Your position in our hierarchy will be very high!"

It does seem a little hilarious that Potbello asks you to find him that candleabra, but that's another clue he was suspicious of the adventurers. Going on a candleabra hunt wasn't going to be Sulman's mission - he was to "Remain in New Sorpigal and remain out of sight. When the time is right you will be contacted and given further instructions. So take heart, and know that our plan proceeds smoothly and we will be ruling this world soon." To speculate again, I can see the irony of the situation - perhaps Sulman lost out to Tanir for the position of Court Wizard. Well, looking at the Kreegans' assumed promises - getting the planet scorched and instating Slicker Silvertongue as king - who better to become Court Wizard now than the last living Wizard? More likely he'll be killed along with the rest of the humans, granted, but come on, the Baa Followers are blind enough to worship alien devils in the first place.

It can be taken for granted that Sulman, like Khazandar in H3: AB, also desperately feared the impending Armageddon and was up to anything in order to ensure his survival (though that wouldn't have gone very far anyway considering what happened to Khazandar). Note Xenofex mentions that "Plan for a New World" - yeah, so come on guys, let's another replica of the Fiery Moon. Naturally, Sulman's ambitious and stupid enough to claw his way through the Baa hierarchy until he hits a safe point where the Kreegans just might spare him from the destruction. Of course the Kreegans NEVER, EVER leave anyone alive long enough to tell the tale, at least not when they're pretty sure of themselves (Roland's rescue), so Sulman was just about asking for it with his fanaticism.

In fact, to start some wild theorising, Sulman hasn't been confirmed dead by anyone. Could he have planted that letter in the adventurers' path, hoping those who found it would unmask Silvertongue and open a position in the First Circle for him? No, that's BS, but it's fun to speculate when the game leaves a few blanks.