Best Attacker by Type (Non-Legendary) guide

1: Use fighters for that. Gardevoir's main job is a dragon slayer. And if you really want to use Gardevoir against dark types, use Charge Beam.

2: Yes. And yes, Scizor (bug/steel), with double STAB moves for both types. Not sure about the other types though.

3: DPS: Jynx, Cloyster, Piloswine, Walrein, Lapras
TDO: The reverse. Except for Cloyster and Piloswine, Cloyster has higher TDO. But Piloswine has the advantage of the ground typing, and a Gen 4 evolution.

4: The list is too long for me to answer it here.

1.
If you really want to use a Fairy, rather use a Pound/DG Clefable.

2.
Yes, Scyther's evolution is Bug/Steel. Flying is NVE against Steel, though.

3.
Highest DPS: Jynx
Highest TDO: (not particularly sure)

4.
https://pokemongo.gamepress.gg/best-attackers-type

Others have answered some of them pretty well.
For Q3: the order in the list (Jynx, Piloswine, Cloyster, Walrein, Lapras) is of DPS descending and TDO ascending. To be specific: (the numbers are from our DPS/TDO spreadsheet, available here: https://docs.google.com/spreadsheets/d/1QMV6Hie9Rdrl3getWRLfdfAS6OqGucMUcc4X97r4QZ8/edit?usp=sharing)

Pokemon #DPS #TDO
Jynx 14 245
Piloswine 11.48 245
Cloyster 11.38 271
Walrein 11.49 317
Lapras 10.47 345

Since the first 3 questions have been answered pretty well I'll have a go at 4.
In gen 4 we get the following:
Fire - Magmortar, 1 more attack over Flareon and more bulk. Could learn FS/OH, which would make it strictly better.
Grass - Roserade, 10 atk over Exeggutor, glassy. Could get Bullet Seed/Grass Knot, or Solar Beam, making it top grass DPS.
Poison - Roserade, 18 attack stat below Gengar, could learn PJ/Sludge Bomb.
Poison - Toxicroak, 50 attack below Gengar. Could learn PJ/Sludge Bomb
Electric - Electivire, 17 attack stat over Jolteon, more bulky. Could learn TS, TB, WC. Easy upgrade.
Electric - Magnezone, 6 attack over Jolteon, more bulky. Has 11 resistances and 1 immunity. Could learn TS, TB, WC.
Ground - Garchomp, 39 attack stat over Rhydon. Has access to good (as far as ground goes) ground movesets.
Ground - Mamoswine, 25 attack over Rhydon. Has access to good ground movesets.
Ground - Rhyperior, 19 attack stat over Rhydon. Can't learn a ground fast move currently in the game.
Rock - Rampardos, 84 attack stat over Golem. Can't learn a rock fast move currently in the game.
Rock - Rhyperior, 30 attack stat over Golem. Can't learn a rock fast move currently in the game.
Ice - Mamoswine, 24 attack stat over Jynx, much bulkier. Has access to good ice movesets.
Ice - Weavile, 20 attack stat over Jynx, also glassy. Has access to good ice movesets.
Ice - Glaceon, 15 attack stat over Jynx, bulkier. Has access to good ice movesets.
Bug - Yanmega, 7 attack stat below Pinsir. Close enough that moveset may make it preferable.
Dragon - Garchomp, 2 attack stat below Dragonite. Has access to good dragon movesets.
Dark - Honchkrow - 19 attack over Houndoom. Could learn a moveset that would make it top dark DPS.
Dark - Weavile - Same attack as Honchkrow. Could also learn a moveset that would make it top dark DPS.
Steel - Magnezone, 2 attack over Scizor, much bulkier. (19 attack below Metagross). Can't learn a steel fast move currently in the game.
Fairy - Togekiss, 12 attack below Gardevoir, bulkier. There is a chance of it getting a fairy fast move (unlike Gardevoir, all of whose possible fairy moves are already charge moves). Would make top fairy DPS if this happens.

Conclusion: Quite a lot of competition in the next generation. Gen 5 will also bring a lot of pokemon that out-attack other non-legendaries until that point, with fire in particular getting 3 new mons with legendary-tier attack stats.

2. Fighting/Steel: Lucario in Gen 4 has high attack but he is clearly less bulky than Machamp