Retaliation does not give extra turns to you. It offers only a counterattack with the default skill. A counterattack is no extra turn for the calculation of buff/debuff duration or any other purpose.

I never tested it at Spider, but I don't see why Retaliation shouldn't work when the Spiderlings attack. With the exeption of Drexthar, who can land HP-burn with his default skill, I don't see any reason to use this set vs. Spider, but technically it should work.

How does that interact with skills that do give an extra turn though, e.g. Robar who gets an extra turn if he places a stun on his A1? I would assume it doesn't actually give him a free turn.

Still, there are some champions that have useful debuffs/effects on their A1, or at least an AoE. If all else fails, it might help them tank if they get an opportunity for an AoE lifesteal attack whenever a spiderling attacks them.