This already has two answers, but I just wanted to add my bit because I've used all three Pokemon before.
1) Scizor: definitely. Scyther has a x4 rock weakness, which means half of its health is cut on a stealth rock switch in. Scizor can make use of Bullet Punch, Bug Bite and Pursuit with its Technician ability, and while Scyther has Technician as well, it doesn't have the STAB priority move that Scizor gets.
There are people that use Scyther w/ Eviolite, but I really don't see the point of a bulky Pokemon with a x4 weakness to anything.
2) It depends; Slaking has found a niche in double battles with Cofagrigus. It's rather gimmicky, but basically you get Slaking to use a weak move on Cofagrigus (that it isn't immune to, otherwise this won't work), and Mummy will take over its Truant ability. The weakest contact move Slaking knows is Smack Down at base power 50.
Vigoroth would only be viable in NU, but it sees some use, especially with decent attack, HP and speed, along with Eviolite defences boost.
However, even with Slaking's Truant, people usually either use a move & then switch it out, or use Giga Impact, which requires a recharge anyway, so Truant and the recharge are both counted in 1 turn.
3) Porygon2 is a good bulky sweeper, and I think it outclasses Porygon-Z, though a lot of people who use Porygon-Z use it as a moderately fast special sweeper. Porygon2 would usually go last in most scenarios due to a base speed of 60 (but this makes good use of Analytic).
4) Chansey, Magneton, Dusclops, Murkrow, Gligar
- Chansey: higher def and sp. def than Blissey but no Leftovers recovery; personally I think she's worth your time because Chansey has reliable recovery moves anyway.
- Magneton: good Scizor counter. With Magnet Pull it will trap other Steel types and be able to deal with them with HP Fire, and as an Eviolite holder it will get a higher chance of survival from fire attacks.
- Dusclops: good UU wall, but outclassed in OU by Cofagrigus (since it has Haze). Base 130 in both defence stats is brilliant, combined with Eviolite boost meaning it can act as the counter wall to the Chansey on your team.
- Murkrow: Prankster Pokemon. Like Riolu it's seeing more and more use with its ability, and Murkrow acts as a Mean Look Perish song trapper. It's a bit gimmicky though, to be honest.
- Gligar: Stealth Rock set up, higher special defence (so more chance of a survival from a non-STAB Ice Beam), and Roost to make up for no Leftovers.
