Excellent AAR, eagerly waiting for the next update 
The Drakes are good, but not invulnerable, archer focus fire can easily bring the down.
By the way : Sir Sloan has access to the almighty Soul Mirror spell.
An equally mediocre item (bracer of ludd iirc)a) What is the "Defeat King Arthur" quest reward? In the Saxons it was some mediocre stat armour I had got twice from trade and a common battle before.
Not really, unless you count what was already available inthe vanilla game (sidhes, teleports ...)b) Is there also an early and cheap to get "win any battle blindfolded" unit similar to the Saxon Archers?
The Drakes are good, but not invulnerable, archer focus fire can easily bring the down.
No, the game flow is the same as the Saxons.c) Are there any real quests which actually utilize the Adventuring ability of your heroes?
Same as Saxons, you can gain heroes via Bishops and Sidhe faction, or through the hero diplomatic panel. However, they are more or less generic heroes, nothing special. All "big names" and new heroes are available with the main quests.d) Is it possible to get extra heroes from the misc factions through diplomacy and what are their stats like?
By the way : Sir Sloan has access to the almighty Soul Mirror spell.