The combo isn't vital to use throughout the whole battle (heck, it's no use against Ciel), and in double battles it will last longer. Earthquake still retains it's original purpose in battle of being coverage, and finding another thing immune to it risks ruining my synergy. That, and Conkeldurr can tank it rather well too.
The problem with getting Starmie and giving it surf is that only Cradily can make use of it/not be affected. This makes it the most useful in double battles, which are rather uncommon in the league, and any other member will not enjoy the surf. I have no reason to use Starmie other than this, because all it'll do is stop me from using the dark type moves which I desire to stop ghost types.
Darmanitan may have surprise factor with life orb, but without speed it won't matter anyways; the gym leaders care more about setting up their own strategies then countering mine, meaning that whatever they're doing will make Darmanitan fall quickly. HOWEVER, in the case of facing a Trick Room team like Noel's, where my speed will be redundant, I will definetely keep Life Orb on Damy here as an option (since items are changeable throughout league).
Oh, and if you're worried about fighting types, my team is more suited to countering enemies with offense than defense. I'll use conkeldurr if I need to take a hit, Acrobatics will DECIMATE them, and fighting types have a tendency to have another type that gives them a weakness (e.g Lucario).
Zoroark is rather frail, meaning that I'll tend to use it for a quick bombing rather than a drawn out war. This means that I don't really plan to use Substitute if it's impersonating Conkeldurr or Archeops, per say. That, and Zoroark is more here to cover my team's weaknesses rather than be it's core, because Illusion can be a little bit shaky to rely on at times.
The problem with using stealth rocks in league is that:
Every team is monotype, so while Stealth Rocks will decimate some, it'll do nothing to others.
In triple battles, switch ins are not as common, and rocks are harder to set up.
Also, Earth Power is needed because it's a ground attack that, unlike Earthquake, won't run the risk of ruining my team (that, and it also covers the special side).