The 0.5s autoshot casting time can take place under the GCD of steady shot (and thus don't have any negative effect on your rotation) if this last one as a casting time inferior or equal to 1s, right?

For exemple:

0s Casting Steady Shot and GCD starts
1s Steady Shot is fired and Auto shot start "casting"
1.5s Auto shot is fired and GCD end

1.5s Casting Steady Shot and GCD starts
2.5s Steady Shot is fired
3s Auto Shot is fired and GCD ends

I'm pretty sure it works like that but i prefer to check with the pros

Correct.
The post I quoted, however, said something about the 0.5 Auto Shot primer taking place when a spell starts, not finishes - which is wrong.

Correct, but I highly doubt you could get this arrangement to work with the gear provided. To get steady shot to a 1s cast, you need 50% haste (1.5/1.5=1). If you have 50% haste and a 1.5 second hasted attack speed, your weapon speed would have to be x/1.5=1.5, x=2.25. There are no level ~70 rare or epic ranged weapons with a speed between 2.20 and 2.40; the closest would be Wrathfire Handcannon at 2.0 (also, The Boomstick, but that has stam/def on it) and Barrel-Blade Longrifle at 2.60. Neither of those would necessarily be a bad choice, but it's probably not worth making your rotation difficult to maintain and use non-optimal gear for a minor increase in damage (especially when superior alternatives are available).

I realize your post was theoretical (regarding the auto shot delay timing) but I thought I'd just throw this in there: The gear available simply doesn't support some of the timing arrangements that we can think up.

Yeah i usually get this kind of haste only when Iaoth and Trophy proc under rapid fire.
Talking about crazy haste effect i noticed something strange from WWS:

13:31'18.484 Corporate's Steady Shot hits Gruul the Dragonkiller for 717

13:31'18.734 Corporate's Auto Shot hits Gruul the Dragonkiller for 717

13:31'19.937 Corporate's Steady Shot hits Gruul the Dragonkiller for 623

I was under Rapid fire, Dragonspine Trophy haste's effect and imp.Iaoth but i believe none of those effects affect the GCD, and still that's 2 Steady Shots in 1.453s, how is it possible?

A latency change of ~0.05 seconds from one Steady Shot to the next, combined with button spamming?

I have been lurking here quite a bit now. Since I am not a "math wizard" I am sorry there is very little input I can give. However, this site has improved my hunter skills tremendously and I thank all those who have given the information in a way that can be put to practical use.

As for weapon speed and haste effects I am wondering how good is the dragonspine trophy for us? I recently specced survival and personally I am finding it easier to weave shots without IAOTH changing my speed. Therefore I am curious on how much the haste of trophy affects dps and in practical use does it make it "as hard" as IAOTH/rapid fire to weave in specials btw auto/steady rotation, assuming you are using sunfury. Or is it more of a "learn to play" -issue?