The stated intent for WOW trash mobs is to extend how long it takes to run a dungeon.
I really doubt that you actually want to do that in D&D.
So, forget about emulating trash. Instead just do a "highlights".
First: hear about the defias.
This is your standard "oh, our farms are threatened by bandits and the king cannot spare troops" type of affair. The PCs are innocently (or not) going about their business and their paths cross with bandits. For some reason, the PCs get involved (TM). They might help the local farmers out of the goodness of their hearts, or they might get told about the regular courier cash run that gets done.
This is one encounter with defias foot troops (minions and skirmishers, possibly with a soldier as a sergeant).
Second: Hear about the secret deadmines.
For whatever reason, the adventurers are after the defias courier, who stops off at all the places the defias hang out at and collects resources/hands out the mail. The players will either capture him, follow him or kill him and hope to puzzle stuff out (make sure there's some clues about moonbrook).
This is an encounter with a small troop of armed guards like the first encounter, plus something interesting in the form of the courier: perhaps some sort of leader type.
Third: Enter Moonbrook. Realistically* this place has to be a working town, because it's keeping a mine/factory running. This has possibilities for deception ("no sir, this is a sleepy country town, not a bandit stronghold") or an infiltration scenario (ie - fortify the town properly).
Either way, this is going to be a skill challenge that may lead to a fight if failed.
Fourth: Enter the mines. The instance has a huge number of miners crammed into a relatively small space. I'd recommend making things more like a real mine, where there are only limited areas that have high numbers of miners in them, and the rest is only barely populated and most likely ill-lit. Additionally a section of the mine is infested with undead.
Again, we're looking at potential combat, but mostly player driven: it's relatively easy to pass through this section unmolested, because it's a working mine: everyone who might be able to hear something is used to hearing the sound of metal on stone all day. I'd expect the PCs will only fight a patrol (ie - a 5 man party of defias troops) if they cause a ruckus AND let people run away to sound the alarm (ie - they're really dumb). Even if a miner spots them, he's more likely to ask questions first, and I'd say the miners are likely minions.
The undead section is seperated from the working mine. I would really avoid giving the players any quests that specifically send them there, but you could plant some stuff there for them to find (loot, tangential storyline). The WoW quests here just ask the question "Why was it hard to find the deadmines when there are living relatives of people who work there?".
Rahk'zor is pretty much a stand up and smash fight. It's Rahk'zor (elite brute) plus a few skirmishers.
Fifth: The lumberyard
I'd probably set up an interesting terrain-based lumber area for this one. Lots of fun can be had with conveyor belts, stacked logs and saw blades. Have a little warm up against goblin carvers so people get used to the mechanics of the area, then go straight into the boss fight.
Give the shredder the ability to fling people around the room to get more mileage out of the terrain. I'd probably make sneed try to flee when his shredder goes down, and fight only if cornered.
Sixth: The foundry
The forgemaster is probably good as an elite brute with an ongoing fire damage attack. Put a group of goblins around him. Make the Goblin engineers minions that create little brutes if they're left alone for too long.
Seventh: A bit more mine, as in 4, but with cannons and powder stacked up all over. Finally there's the big locked door, and the obvious solution.
Eigth: The pirate cavern. Fun with water and the docks. Have probably only one pirate encounter, then Mr smite.
Mr Smite is hard to implement: noone is going to like AOE stuns that automatically hit. Probably the best solution is to give him an AOE knockback that he can use as a minor action. Have it recharge on 5-6 or bloodied.
It would be good if the weapons weren't sitting in a chest either. It's more interesting if the PCs can take some sort of action to prevent the boss from repeating a pattern: hang them on a wall or on a weapon rack in plain view and make them fun or interesting in some way. The WoW Smite fight places the emphasis on the stun: the weapon changes almost have no effect whatsoever.
Then you have exploring the dreadnaught. The cannibalistic cook is an optional fight somewhere down in the galley.
Somewhere we have captain greenskin around. If he's still alive when the van cleef fight starts, then it's likely that he (and any pirates still aboard the ship) will join in. Killing him will mean the pirates will not necessarily organise themselves in time to take part.
I'd probably scatter the defias enforcers throughout the ship too, giving the PCs the chance to take them out if they explore.
Finally the showdown with Van Cleef happens. I'd have the enforcers arrive at a rate of 1 per round, with Greenskin and a gang of pirates showing up 5 rounds in assuming that an alert is raised.
That's going to be a really tough fight if the PCs just head up to the bridge and start a slog fest. But unlike WOW, it's entirely possible the party can get the jump on VC and do something clever, like drag him overboard and take the fight to the water, make use of some of the cannons on the ship, cause a commotion that distracts everyone etc etc, and I think that without any of these, this should be a really, really hard fight.
All up, the PCs will have 10 encounters if they don't go looking for trouble, and do the VC fight all in one hit.
More likely a group will have around 15, avoiding optional fights and splitting up the VC fight a bit.
If they explore every nook and cranny, they might hit 20 encounters.
You probably want to hand out full XP for avoiding optional encounters.
