1. The official timeline names "Sylath" as the God of Air whereas Dark Messiah give us "Ylath". Which name is correct? Also a "Mal-Beleth" is mentioned as the Sovereign instead of Kha-Beleth in the official timeline. Is this a typo or are they different characters?

For the record, the names of these entities are officially "Ylath" and "Kha-Beleth." However, as Ashan is a world with many races and cultures, it would not be surprising if there were local variants of some of the names. Just as English speakers call the capital of Russia "Moscow," the French call it "Moscou," and an accurate phonetic spelling would be "Moskva," it would not be surprising if there were slight variations in some of Ashan's proper names from nation to nation.

2. What is Raelag up to in the 19 or so years between HoF and DM?

Much of Raelag's time was spent in studying the ways of the Dragon Knights and entering into the order. There was also a lot of exploring and discovery to do; the Dragon Knights as well were on the tracks of things such as the Skull of Shadows and the Dark Messiah. However, on a purely personal level, Raelag also had a number of things to work out in tracking down Isabel, freeing her, and trying to work out the complexities of a long-distance relationship... Raelag is a character that we like, and it wouldn't be surprising if he pops up again from time to time in the world of Ashan.

3. How exactly did Raelag end up with the true Isabel?

It may not have come through in the final cutscene, but Raelag was the only one who realized the switch that Biara had pulled. He stayed in Sheogh to free Isabel, and together they fought their way out of the demon world. Exits from Sheogh are neither stable nor predictable, and getting out and making it to the Griffin Empire was a long and challenging process.

...how come the other three heroes who invaded Sheogh were clueless?

They weren't watching her as closely; they weren't in love.

4. Is Arantir dead or alive as of DM's ending?

I think we can assume that his defeat at the hands of Sareth combined with the cataclysm at the necropolis would stretch even his considerable abilities...

5. Was there a specific reason why Elrath intervened and killed Laszlo in HoF, beyond "Godric's devotion"?

At the time of Godric's death, he was far and away the most loyal and devout paladin in the entire Griffin Empire. Ashan is a place where gods are present and alive; Elrath would certainly have heard the dying wish of his most faithful and unswerving believer.

6. Where are Aidan and Nadia throughout all of H5's timeframe?

Nadia, sadly, died in childbirth having Zehir. That was one of the reasons for his unusual personality, and also the reason that Cyrus grew increasingly cold and bitter over the years.
Aidan became a Dragon Knight, and had other wars to fight, studies to pursue, and evildoers to vanquish. He also became acquainted with a certain Tieru… But that’s another story.

7. Which of the four DM endings is canonical?

The official answer is that we'll have to wait until a game or a novel comes out that happens right after that event! Personally I prefer the idea that he made his decision by refusing to decide; that he kept the Skull of Shadows for himself and the fate of Ashan hangs in the balance as Sareth ponders what future would be best. Heroes Kingdoms explores another of these possible endings, the one where Sareth obeyed Kha Beleth and unleashed the Demons.

8. If Sar-Elam was human, why is his skull draconic?

Sar-Elam began life as a human wizard. In fact, he was the first and the most powerful one that ever lived. As he transcended the gap between mortal-human and immortal-dragon in becoming the Seventh Dragon, he also gained the ability to change into a draconic form.
When he died in the process of creating the prison world of Sheogh, a fragment of his soul was split and bound to a huge chunk of shadowsteel ore, that was later shaped like a dragon skull to honor his “divine” (draconic) essence.

9. Is there a backstory to the Solmyr and Crag Hack in Ashan (mentioned by Narxes and Isabel in the H5 campaigns), or were they really nothing more than a bit of name-dropping?

In a world of millions of people there are many, many names. Some of them could simply be coincidental; others have such power that they may reverberate across all the possible worlds of reality... But to give you a more straightforward answer, yes they’re part of Ashan lore and you may meet them one day.

10. "In search of power" (a HoF single scenario) revolves around Veyer trying to "create" a new Demon Sovereign because the previous one is apparently dead.

First of all, Veyer is not the brightest light on the Infernal Christmas tree and his interpretation of events should always be viewed as unreliable. Secondly, when a demon is 'killed' it enters the demonic cycle of rebirth; it maintains its old memories but is reborn as a lowly imp and must re-ascend the ladder of power (preferably by stepping on all those beneath it). Thirdly, there is, was, and will be only one Demon Sovereign so Veyer's ambition is wildly optimistic. The questions in this case are more like: Why does Veyer believe that the Demon Sovereign is dead? During which of Veyer's "lives" does this take place?