Update
Since Patch 1.03 the bug of randomly getting human form back after multiplayer sessions has been fixed. According to the patch notes, you now get some humanity back for helping others as a white phantom, meaning your health penalty will decrease. When all your health is back, you will regain your human form (I have just tested this). This means you are no longer dependent on human effigies to get your humanity back in online mode.
Patch note excerpt:
Players now receive a small portion of Humanity after successfully assisting in a Multiplayer session instead of regaining full Humanity.
Before Patch 1.03:
Before Patch 1.03, you got revived by playing co-op at random. It does not matter whether you defeat the boss or die, sometimes you will return to your world revived, although you placed your summon sign hollowed.
There are several rumours on the internet, that it gets less likely to revive the more often you fight the boss, or that the amount of human effigies you have has an influence on your chances to revive, but they are wrong.
I once got revived although the summoner died in a boss fight, and once it took six times killing a boss and then only one time killing the same boss to revive, although the circumstances had not changed.
This random procedure indicates, that you are not actually meant to revive. In From Software's twitter post, that has been partially translated at reddit, it has been confirmed, that phantoms reviving is a bug and not actually supposed to happen:
Somebody asked the official Japanese FromSoftware twitter if it was a bug that sometimes you didn't revive to human after helping somebody else play coop.
The answer? "Actually you're not supposed to revive to human, we'll investigate why that happens".
Human effigies are limited in Dark Souls 2, as long as you don't use Bonfire Ascetics, which practically turn areas into New Game + difficulty, but as long as you use them rarely and with care, you shouldn't run out of them. You can find a lot of them as treasure and they also drop from (limited) enemies. For more information on that, see How do I farm Human Effigies?.
Should you find yourself running out of effigies anyway, you can either try to find the Ring of Life Protection, which makes you practically immortal as it can be repaired, or you could try to get summoned and ask other players through private messages to give you an effigy.
What the Giant's Kinship actually does (activate the Golems? Open the Throne?) is a bit of a mystery, but what we do know is that you can't access the throne without it.
Along the way, she tries to direct you a bit. When you talk to her after gaining the King's Ring she says:
Brave Undead, seek the throne. Follow the symbol of the monarch, and do what must be done.
After you meet the Ancient Dragon, then go back and speak with her again:
Brave Undead, what did that dragon tell you? That thing is a prop, a false deity. Don't be fooled, my Undead.
If you go back and visit her after you gain the Ashen Mist Heart, she says:
Seek the forest, you, where those wretched Giants slumber.
She leads you on to try and gain the Giant's Kinship. She knows you need it to gain access to the throne.
WARNING - SPECUALTION AHEAD. PLEASE TAKE CARE
I feel that Nashandra is attempting to pull your strings through the entire game, making you feel as though she wants you to become the new King of Drangleic. However, shock! Horror! She was playing you all along!
It's pretty obvious we can see that happening once we've beaten the game once round already.
END OF SPECULATION ZONE. TAKE CARE AND HAVE A NICE DAY
Basically, the reason that she leads you on (so horribly) is that either she can't or won't get these things herself, (most likely can't, because Vendrick hints that he attempted to seal the way from Nashandra, and the Ancient Dragon seems to know a thing or two about her true nature as well) so she gets you to do all the heavy lifting, then attempts to snatch the victory from your hands, just when you were so close!
Best Answer
In this Dark Souls 2 Wiki it says that sometimes it costs 0 souls to be granted a pardon.
On that same wiki but on the page about sins, it states that this is a glitch that seem to have started when the Crown of the Ivory King DLC was released:
I'm guessing this is what happened. To verify, all your sinner statuses should be reset if this is the case.
I also found a Steam Post of a player talking about the same thing and some people have mentioned that it is a glitch that seem to have occured with the DLC being released. Since you have the Scholar of the First Sin edition (at least you have it tagged), you have the DLCs, and I'm betting that glitch occurred.