respec

has anyone already done the quest to respec 2nd profession?
if yes, is it repeatable? (should read "can it be used for skills-farming?")

not yet but here is the info

The bottom line is that you could have a primary character who has experimented with all five available secondary professions. That character will likely have good stock of skills in each of those lines. In fact, that character could have every skill in each of the five available secondaries. The character then, in ways not yet to be revealed, will be able to shift to a different secondary and take advantage of a long history of roleplaying character development.

I think I'm missing something. When I tried another secondary profession in the game beginning, I lost any skills from trying a previous secondary profession. Are you saying it should be possible to retain all the skills from each profession you try? How?

no
they wont be on your char, but they will be stored in "unlocked" - so u can use them to make PvP chars

good good good

And here comes second question then - can i return to some professon? (say I am warr/necr --> respeccing to warr/elem --> grabbing skills --> back to warr/necr for item farm reasons)

this is after the academy
you are in the tutorial until the academy
you cant switch until you reach level 20 so be happy for awhile with what you have