Personally, I'm not really sure Scizor needs any investment in speed, atleast for the Choice Bander. The 8 EVs listed in the analysis only allows it to outspeed 4 Spe Vaporeon and I'm really not sure it's worth it. Also, the Discuss My Pokemon Thread is here.
You can steal neither Swalot's Leftovers or Muk's Black Sludge, as both pokemon have the Sticky Hold ability. No other elite four pokemon have items, as far as I am aware. In response to your other question, yes, you can steal more than one Leftovers from Snorlax, one for every Elite Four win, if you catch it, you're done, so be wary.
Only a difference in appearance, nothing more, nothing less, though if you want to go really in-depth there's probably an immeasurably miniscule effect psychologically. Good look abusing that.
Probably WishBliss since most of your team won't have recovery but the Cleric could be useful if a lot of your team is succeptible to status. I've heard people don't use things like Blissey on an offensive team because it slows down the momentum.
There aren't any 100% counters, I don't think (there rarely are to things that can explode), but there are loads of checks; anything that can outspeed and OHKO a Wailord (given that Wailord is generally scarfed, you'd need something like an Electrode (where any common build can OHKO the Wailord after stealth rock), or a scarfer of your own with a decent super-effective move) can beat it if it switches in safely, and of cause ghosts won't care about being Selfdestructed on. You might also want to look at Skarmory (especially specially defensive versions); it hardly takes any damage from exploding Wailords, and can 2HKO with Brave Bird (severely reducing Water Spout's damage in the process), although even specially defensive versions are 2HKOed by Water Spout if the Wailord's taken no more damage than Stealth Rock on the switchin. Vaporeon is another possibility (immune to Water Spout, and it will survive a Selfdestruct, despite being badly injured in the process).
You could screw with their abilities with Skill Swap, Gastro Acid or Worry Seed and then swipe their stuff.
Holy shit I might try this, it saves me playing through Sapphire a bunch of times and gives me a reason to blow off revising and train a bunch of random Pokémon.
well, if any pokemon is withdrawn, pursuit would still hit that pokemon x2.
Unless it's changed from ADV, you can't gain exp at Lv. 100 and thus can't gain EVs from battle. You may be able to gain them from vitamins, but normal vitamin rules would apply.
Defensive Empoleon should work pretty well since it has good SpDef and resists Water Spout and Self Destruct. It also has Grass Knot which will hit Wailord hard. Vaporeon and Blissey should be able to switch in on aything but Self Destruct and can possibly use Protect to avoid it if Wailord doesn't have a Choice item.
