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.
If you are not bypassing the spell prereq for the item, you are forced to 'cast' it.
http://paizo.com/prd/magicItems/magicItemCreation.html
From the creating wonderous item section (similar text can be found in almost all other feat specific sections).
If spells are involved in the prerequisites for making the item, the
creator must have prepared the spells to be cast (or must know the
spells, in the case of a sorcerer or bard) but need not provide any
material components or focuses the spells require. The act of working
on the item triggers the prepared spells, making them unavailable for
casting during each day of the item's creation. (That is, those spell
slots are expended from the caster's currently prepared spells, just
as if they had been cast.)
so in your case, each day you spent crafting the belt, you would need to use a copy of the scroll or have someone around who can cast it for you. Otherwise, you have to add +5 to the DC of the check.
Best Answer
To quote the creation rules you cited:
The rules here clearly state that yes, the necromancer could fulfill the requirement needed for Necromancer's Athame if the necromancer had Craft Wondrous Item. What they are less clear on is exactly how this would work.
Helping someone work on a magic item would be an "Aid Another" action to influence the outcome of this final skill roll.
Here are the complete Aid Another rules:
First, notice that you are required to make "the same kind of skill check in a cooperative effort." As a GM, I would rule that this means that all participants (the creator and the aiders) must work on the item 8 hours a day (or however long the creator is working) throughout its entirely. The aiders cannot simply show up at the end of the creation and aid for the final check.
Second, since a "cooperative effort" must be made, I would rule that the creator must expend as much effort as working unaided. This means that paralyzed characters cannot craft magic items, unless they themselves have some other way to work with the necessary tools.
Third, rules-as-written states "you can't aid another to grant a bonus to a task that your character couldn't achieve alone." This would mean that every character involved, be they creator or aider, must be technically able to craft the magic item themselves. However, note the following lines from the creation rules:
Taking this into consideration, every character who can make a spellcraft roll and who has the requisite item creation feat can aid in the creation of a magic item. Each aider would fulfill any requirement they could, and add +2 to the creator's skill check if they succeeded in their roll.
For each prerequisite not met by the creator or any aider, increase the DC by 5, then roll the creator's skill check, adding to it for each successful aider.
To address your final question about the number of people working on a magic item, the rules for Aid Another leave this up to the GM. I would argue that the number of people lending aid is proportional to the complexity of the item. A scroll, ioun stone, or wand, for example, may not have any room for aiders; they would just get in the way. An Apparatus of the Crab, on the other hand, may have a dozen people working on its many parts.