Why Dori in a Dale deck? Well, why not? After Bard son of Brand and Brand son of Bain you can use pretty much anyone as the fourth hero. The main thing is a bunch of Dale allies and a bunch of cheap attachments to power them up. Although Warrior of Dale is IMO the least impressive of the initial Dale allies (I blame Bow of Yew for that), he is on the expensive side so extra Tactics resources go to waste. And a great defensive hero can keep the wolves at bay until Redwater Sentry on line.
Now, Dori unfortunately isn't a great defensive hero, with a 2/5 defensive split and no traits but dwarf. But a fellowship with Dale doesn't need a permanent defensive hero, since the Sentry with Hauberk of Mail and Raiment of War is a 5/6 sentinel defender. Dori can take some early hits, and can help Bard, Brand, Celeborn (in the other deck) defend for 4, and help Radagast (in the other deck) defend for 5. None of those heroes have sentinel, but with Dori around there's several heroes in each deck that can provide solid defense with each deck, before the Sentries come online.
Playing Dale is straightforward -- get King of Dale in play, get discounted allies out, play cheap attachments and draw card. The power of a Dale deck steadily grows over a quest. In this case the wrinkle is that this is also is a support deck for the Play-Gandalf-Every-Deck that needs six resources. Steward of Gondor can provide two of those, and having this deck provide it makes it easier for the other deck to get the other combo pieces together. Magic Ring could also provide a resource, as could Errand-rider -- later in the game, Traffic from Dale can generate a ton of resources, might as well give them away.
The only attachment that goes on poor Dori is King Under the Mountain, which also can provide extra card draw if it comes out.