I think the Irontooth encounter is a must.
1.) Unusual Kobold Activities are reported. Characters investigate, find the Kobold lair and fight Irontooth and his Kobold underlings.
They find evidence of something going on in the Keep.
2.) The PCs locate the Keep and have to fight a mix of Goblin and Hobgoblin guards at the entrance.
3.) The PCs stumble upon some undeads that protect the passage to the second floor.
4.) The PCs might find the water monster and the dead slavers, and maybe some Kruthik. (I would put this in because the monster is unusual and interesting, but it alone is probably not enough against the PCs.)
5.) The PCs encounter Kalarel.
If you want to avoid most of the dungeon crawl, you might want a different approach. Cut down the encounters in the dungeon to only two encounters.
1.) Overland Encounter. Ambush by Kobolds.
2.) Kobolds lead to Irontooth.
3.) Irontooth leads to Dragon Graveyard. Some ruckus at the Dragon Graveyard. Goblins and Hobgoblins here probably, maybe with a few Skeleton Minions. They try to dig out something important for Kalarel. This might mean that Kalarel cannot finish his task - but maybe it doesn't, it's best to take him out.
4.) Undead protecting the Keep Dungeon entrance.
5.) Kalarel.