So I've been searching now for hours, in the game files (with s3pe), here in the forums …
I can't find the ressources that manage 1) which trait will spawn which lifetime wish, and 2) how much lifetime happiness a lifetime whish will give.
I know there is some XML with that information. Velocitygrass does mention that on the simswiki. But he hasn't a current profile or talk page there (and I don't know when he was last active), so I can't ask him directly.
Somebody knows?

I only did a quick research, so maybe not everything I say is accurate.

Lifetime Wishes are Dreams categorized that way. I think they are also called major dreams.
# <Category>Lifetime Wish</Category>
There are also these 2 other tags that seems relevant, not sure if all 3 are required to qualify
a dream as Lifetime Wish (I guess not, judging by the link I put below)
# <IsLifeEvent>x</IsLifeEvent>
# <LifeEventIsLifetimeDream>true</LifeEventIsLifetimeDream>
You'll find them in the DreamsAndPromises _XML resource (GameplayData.package).

Now to your questions:
1) Traits related to dreams/wishes seem defined in DMTR resources and are linked with the dreams
through the PrototypeId field (that contains the value of the Id of the dream node primitive).
2) I haven't searched very carefully, but I think the happiness gained is the result of a base score
(called fulfillment) to which various modifiers are applied (see DreamsAndPromisesManager _XML)

Example:
Lifetime Wish: about reaching a certain net amount (sorry, don't remember/know the exact English name) [resource DreamsAndPromises, id 2004388412]
Traits associated: ambitious, mooch, kleptomaniac, snob [resource "Cas Major Dreams Feeder"]

Other info you may find interesting: http://simswiki.info/wiki.php?title...:Sims_3_New_LTW

Wow, I think, this helps a lot (even if it leaves a whole lot of research ).
Funny that the provided link is by the same person on the wiki I did mention …

edit…
After a bunch of work – first extracting the IDs from one XML, then searching those IDs in the other DMTRs and get the associated traits – I have the information I wanted.
Thanks again.
How could I/one mark this topic as solved?