Before we look at the army, yellow alliances need a Hero of Valour leading each faction, so unfortunately you can't ally in just a Wild Warg Chieftain with Moria.
If you want to use the Cave Drake instead of the Dragon, that's not a bad army either. I would swap out the Wild Warg warband for something like Durburz and 11 Goblins.
As for your other questions:
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Dweller seems like å bårgåin compåred to the cåve troll?
They're cheaper once everything is equipped, but it depends what you're after. Dwellers are harder to kill, faster, and have a higher FV, absolutely. Trolls are fantastic at killing things, they have S6 and Burly which is terrifying. Trolls also have a smaller base, which means you can maneuver them better. Both are valuable, usually it's a matter of personal preference or what you think your army needs.
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Åre wårgs useless?
I wouldn't say so, no. They're fast and cheap. Don't take too many of them because they are really easy to kill, but as long as you have a purpose for them in mind, they can be valuable.
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Cån wårg chieftåin knock ånyone down with S6?
No, it's not a cavalry model.
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Cån drågon model be used ås å dråke (looks best)?
That depends on your opponent. Most tournaments will say no unless you've done a conversion, but in a casual game most opponents will be fine with it. Just make sure you clear it with your opponent before the game.