Nile, the whole point of serperior is contrary leaf storm, the 140BP Nasty Plot. Also, leaf storm Halves special attack each time it's used. Doubles with contrary.
If they use something to disable contrary, a) it'd be temporary, b) you'd still get one hit in at full power, or, if you've already boosted, you've got all those boosts to lose before you're back to being normal. Gastro Acid disappears on switch out, as does Mummy, I believe, and I'm not sure if there are other things, but there's no permanent ability removal.
Yeah, Stick with Leaf storm, and hyper beam I guess, seeing as you seem set on that anyway, and yeah, it is really the only option to avoid sap sipper, wring out and round are the only other non-grass special moves it gets.
DEFINITELY use Giga Drain, though, not only is it a great move, but you can get back a fair amount of health. It's 75 Base Power now, so it's a decent move when Leaf Storm runs out
Just keep it as backup. Why not?
And, unless you're lucky enough to get a good strong Hidden power fire or Ice, you've got a spare slot... Just realised the glare issue. Might as well just put on Dragon Tail (extra move slot, might as well fill it), or if your Hidden Power works out at reasonable power and isn't grass type (use an online calculator, I guess), use that.
The other options are Toxic and Leech Seed, not much use on a Choice set, but yeah.
The main point of Dragon Tail is getting a small bit of free damage on the opponents counter, but MAINLY scouting what that counter is without letting it hit you, giving you more information about their team
The point is, even if it seems that leaf storm and hyper beam do all you need, you might as well fill those two slots, you've got nothing to lose, and you'll have more PP to spare, and a way to restore health.