Is it supposed to work against effects like glitterdust and hold person which grant an additional save each round?...
OR
Is it also effective against effects that only have an initial save, including poisons, diseases, ray of enfeeblement, color spray, etc...
creature with this boon can, as a swift action, make a saving throw against a single spell or effect it is suffering from that grants a save
Tricky. My guess is that the affliction needs to require additional saves for this ability to work, but the text is unclear. It does say "grants a save" though, not "granted a save". But that's undermined by the fact that it's using Liberty's Blessing which causes a save to be made, not the malicious effect.
If it does work that way, does it allow you to roll saves vs effects with a duration of "permanent" such as "bestow curse"?
If indeed the ability grants a save even when the effect no longer can be saved against, then yes: it would work against Curses.
Overall, if the domain power works in the second way isn't it a little too powerful for a lvl 1 ability?
Yes, and no. It's awfully potent, but shouldn't really come into play much at 1st level anyway. So it being a first level ability is somewhat irrelevant.
Liberty's Blessing (Sp): You touch a willing creature as a standard action, granting it a boon. A creature with this boon can, as a swift action, make a saving throw against a single spell or effect it is suffering from that grants a save. The DC of the saving throw is equal to the original DC of the spell or effect. If the saving throw is successful, the effect ends. This boon lasts for 1 minute or until successfully used to remove a spell or effect, whichever duration is shorter. You can use this ability for a number of times equal to 3 + your Wisdom modifier.
How I play it: Liberty's Blessing allows a person to use a swift action to make a saving throw to remove the effects of a ongoing malady.
This allows you to essentially make additional saving throws against ongoing effects such as hold person, entangle, etc. (The spell allows a saving throw as normal, and liberty's blessing will allow you to make another one as a swift action if necessary.)
Curses, Petrifications, Polymorphs, etc. once the save is failed, no longer grant a save, which means liberty's blessing is of no use.
Very Respectfully,
--Bacon