I confirm that the Arc Gift worked on my Klingon and Orion KDF toons. You may have to click on the "Head" button, then select Off-Duty from the drop down menu. From there, click on the Uniform button and then select "Off Duty - Bare Chest".
Member since November 2009... I think.
(UFP) Ragnarök Fleet - Logistics Division Vice Admiral T'Phira; Captain Selena; Captain Altecha; Captain K'Kera; Lieutenant Aydihe Tefx; Subcommander S'ena
(KDF) Lieutenant General Vokno (aka Lady Vokno); Lieutenant General Raylene
I don't think that sto would be stupid enough to make arc mandatory, too many people are against it and would stop playing until a work-around was found. As in many people are choosing to NOT install arc already. Myself included. I say this after having invested more that $1k in the game so far, with plans to continue paying to win. Provided I can work around arc.
If you did the claim and uninstall thing, some people are finding what's supposed to be an account unlock is acting like a character unlock and you need to have logged into each character separately while launching from Arc, not just log in once and be done with it.
Things is that costume slot bug prevents kdf players from having off-duty uniform (second and third slot are not usable and first slot is only for uniforms). Maybe those kdf players that you saw bought more costume slots so bug is not on those slots - not sure.
If that is the case, then the outfit will only be available to characters that unlock it before Arc becomes mandatory. Consequently, some of my characters may eventually find themselves unable to access it anyway.
However, evidence suggests that any such items always end up being account-wide. Tribble testing rewards, all of the giveaways so far...
Edit: The IDIC tribble was one exception I cannot explain - but that went into the C-Store afterwards.
ARC is not mandatory, and will not be mandatory for Steam for example.
My testing has shown me this:
Launched under arc, first character sees email and eventually saw the costume
KDF character launched under ARC, saw costume bits but didnt create costume.
relaunched character without arc, couldnt see costume bits
re-relaunched under arc, saw costume and created/saved it
re-re-relaunched without arc and could still see costume and wear it
So basically somehow ARC is passing info that you are using it and unlocking the costume
and if you dont save that costume, it treats it like it doesnt exist any other time.