^ Waterfall is better for its perfect accuracy and 20% chance to flinch usually. I don't think you'll be missing out on key OHKOs or 2HKOs from using Waterfall over Aqua Tail.
As for Lucario vs Excadrill, I'd Aura Sphere right away if I know that Excadrill was gonna stay to Rock Slide. If the opponent has anything decent to stop Lucario, he can simply switch that in to take the Vacuum Wave, saving Excadrill for later.
LO Darmanitan seems to deserve more use over Scarf Daruma in the sun. For me, the LO + Sheer Force synergy is wasted by Darmanitan holding a Scarf. Yes, Darmanitan can double up as a revenge-killer / scout, but doesn't a Cholorophyll mon do that already? Scarf Daruma seems analogous to Scarf Haxorus - it deserves more power. Granted, unlike Haxorus, it is hard to find anything that can survive a Sun + Sheer Force-boosted Flare Blitz, but I think I would prefer the flexibility of LO, not being forced out by Heatran or Dragonite, but rather finishing them off with a successive Superpower / Rock Slide. I'd also rather not spam a recoil move if it's unnecessary for the kill.
If Band Haxorus can pull it off, then there's no reason why Darmanitan can't. With 95 base speed and Jolly Nature, it will be faster than a non-scarfed Rotom-W, Band Haxorus, Mamoswine, Dragonite, Gyarados, Excadrill (in sun) and Modest Celebi, so it still has a good potential to sweep without Scarf. With LO + Sheer Force + Sun, it should be doing more damage than Haxorus, despite being Jolly.
I think a simple set of Flare Blitz | Fire Punch | Rock Slide | Superpower @ LO, would wreck shit up.
EDIT: Admittedly, I may be grossly underestimating the power of Scarf Daruma