First doubt: Can i make 2 atacks with my thirsting blade invocation and use the greenflame blade? Can i use once and the other atack is normal? Or i can't use this and atack twice?

Casting green flame blade is an action which involves making a (single) melee attack. Thirsting Blade allows you to make two attacks when you take the Attack action, but making an attack is not the same as taking the Attack action.

So the answer is no. When you cast green flame blade, you are only ever making a single attack as a part of casting that spell.

Second Doubt

You get class features when you get them, and cannot delay a choice about which option to receive.

Third Doubt: Hex and Hexblade Curse can happen on the same target? Can i "double curse" a creature? Hexblade Curse feature does not say it's concentration or anything, so, yes?

Correct.

Fourth Doubt: If i land a critical hit using the Greenflame feature, the d8 from it crit as well? Eldrich Smite crit too?

The you get the "extra damage" dice from green flame blade and Eldritch Smite when you score a critical hit when using them. If the green fire leaps to another target, that damage is not affected by the critical.