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.
According to a Mike Mearls tweet, the minimum for your Barbarian to receive is 8, not 12, and 'roll' in the text refers to the total.
Whilst trying to get my head around the mechanics of my other answer, I came across this collection of Mike Mearls and Jeremy Crawford tweets.
The question of the durable feat is specifically dealt with (M = Mike Mearls response):
There has been confusion over the durable feat and the meaning of "roll" does a 20 con wizard regain a min 10 or 15 hp w/ feat?
10 - roll means die roll + mods. Anything that affects only the number yielded by the die will say so specifically. -M
I know I've given an answer to the contrary, but I won't delete that answer until I see what the community thinks, as I'm not sure how much store we (should) put in such tweets. Even their compiler said:
The intent of some rules is clarified well, while others are "clarified" in precisely opposite directions by each designer.
Best Answer
The Servo Crafting feat enhances find familiar, the Quicksmithing feat does not
It is true that you could pick find familiar as one of your ritual spells via Quicksmithing:
However, Servo Crafting adds:
Normally, the find familiar spell does not allow you to have essentially infinite range on telepathic communication and seeing through its senses, speak through it with your voice (I'm ignoring Pact of the Chain warlocks here) or forgo an attack to allow it to make an attack instead1.
Since the Servo Crafting feat enhances the spell in this way, it is distinctly different from simply picking find familiar via the Quicksmithing feat, which does not enhance the spell in this way.
1 It can do something similar, in that it can allow you to deliver a touch spell through it, but that it quite different from what Servo Crafting provides: