Well, I cannot be of much use in this matter since I changed the background of the adventure to Forgotten Realms and instead of Y being the patron of the Maure family, I went with Malikizid as the patron. And I changed the scythe to a longsword that the Branded King (Malikizid) used when he was still a Solar in service to Corellon. Of course, during the Crown Wars, as a reward to the elves who worshipped him, Malikizid (I think I am spelling his name right but I may be wrong) gave them (the Maures) his longsword which since has become corrupted by his fall from grace. And the longsword can be used (like the scythe could) to summon the Branded King fully in the Prime.
That's how I handled the problem with the scythe.
Your idea of Y using the scythe to bind the King of Ghouls to his service once more is a good idea. Instead of summoning Y, the ritual summons the King of Ghouls (who can be an Epic level challenge or a CR 20 challenge). Perhaps the party has to use the ritual of summoning because maybe the King of Ghouls knows Y's truename and the pcs need to stop Y from unleasing a horde of undead and gnoll berserkers and other assorted demons on the Prime? There is a lot of funky ideas out there.
I tied the story of Maure Castle to the story lines from the Lost Mythal novel series because (a) I enjoyed the novels and (b) my players' pcs were all elves who had adventured in Myth Drannor. It was simply easiest that way for me to hook them in the adventure.
CB out.
There's really no reason to change anything. The weapon is Intelligent and works to further and protect the interests of Yeenoghu - not the Ghoul King, Doresain. As for outdated information with regards to the relationship between Doresain and Y, well we're talking about elements of a treasure room built hundreds of years ago. Then, the relationship was most certainly current. How on earth would they have known of Doresain's current break from servitude? And who's to say Y won't just bring him to heel again? Besides, it's just a statue of Doresain that animates to hand the Scythe of Yeeonghu to someone that comes within 5 feet of it. Your players need never know that the statue is more than a hideous ghoul effigy. It's just minor stuff that really doesn't need to be addressed since it effects absolutely nothing.
OK. Let me correct myself on that last sentence. It's minor stuff, but very cool history to let your players in on. It's also nice to show off your DM knowledge of the current situation between Doresain and Yeeonoghu. Heck! You could even let the players get some notion in their heads about using that knowledge against both Y and the Ghoul King. Who knows what it will lead to. Players are such unpredictable creatures.