[RPG] Post character creation, getting spellcraft bonuses

Do its bonus stack with Aid Another's not-circumstancial bonus ?

Yes. They are bonuses of different types, and therefore stack. Untyped bonuses always stack with all sorts of typed bonuses, and also with most other sorts of untyped bonuses (the exception being that a bonus will not usually stack with itself if you would be entitled to it more than once, even if the bonus is untyped; this is known as the Same Source limitation). They could have made it an exception if they wanted by explicitly saying it does not stack with Aid Another, but that would be pointless. Thus someone using Aid Another while having the feat effectively doubles the bonus that they offer.

What does mean "your assistance doubles the gp value of items that can be crafted each day” ? Do you craft twice faster (thanks to a very twister way of reading "double the gp value you can craft each day") ?

That’s precisely what it means. Actually, you craft a little more than twice faster because you are hitting a higher DC thanks to the feat’s +2 circumstance bonus, but that particular line doubles your speed compared to if that line was not there. Your progress on any crafted item is measured in silver; when the silver value of your work crafting the item is equal to the value of the item in silver, you are done. If your crafting checks produce twice as much silvers’ worth of progress, then you will meet the item’s value in half the time.

Why is Cooperative Crafting a feat?

A good question; to my mind it’s not very good design. Aid Another was already sufficient, I think, to model someone helping you out. If anything, I would have made this sort of bonus contingent on hitting higher DCs with your Aid Another roll (since, as you say, DC 10 is remarkably easy to hit), and for having both people have the relevant feat. This seems like an unnecessary feat tax to me, even if the effect is reasonably good.

On the other hand, I’m not too familiar with Pathfinder’s options in this regard, but if there are enough effects out there that stack and reduce crafting times or crafting costs, that can lead to very overpowered characters if they can get them all (see 3.5’s Artificer). Which makes me leery of an effect that doubles crafting speed, particularly when you don’t need to take the feat yourself; a cohort could take it, you could hire an NPC who has it, or whatever. Alone, this feat is fairly week. In combination with other, similar feats, it might be overpowered. Which again would just lead me to wanting to have a lesser effect, achieved just by hitting higher DCs than 10 on Aid Another.

Item's generally don't have properties to boost accuracy

Weapon accuracy is tied to 2 things inherent to the item itself: Weapon proficiency bonus and magic item enhancement bonus. Since you've gtot inherent bonuses on the magic item enhancement bonus doesn't matter and since you are using implements, there is no proficiency bonus to tap into. 4e really relies on feats and stat mods to boost accuracy for your PC.

Feats: Accuracy > Damage > Defenses in 4e

You've probably started to get a feel for this as you miss on your (sure to be) damaging attacks. This is accuracy feats are always stressed before damage feats, you generally do half or no damage on a miss which can be quite a big deal. If at all possible talk with your DM about retraining some of your feats now. Those defenses feats and daul implement caster are nice, but you'll be killing the monsters much quicker if you hit (and thus also avoid damage) I would drop both improved defenses and unarmored agility (and pick them up later in heroic as you level) and instead take the implement expertise for whichever you are using (dagger implements I assume) followed up by a superior implement expertise feat (accurate). Those two feats will boost your to hit by another +2.