You find out in exactly the perfect time you need to know ... unless you attacked Dror Ragzlin and Gut allready. laugh
But if you talk to them, fiding what they can offer you, exactly as the evil character should have ... you find out about this possibility in best moment, meaning exactly the moment you created it.
I think you mean to say "the last possible moment". Why would I just walk up to a ton of very-likely-to-be-enemies to see what they have to tell me? The perfect time to know that a transaction can take place is
before entering the metaphorical dragon's lair, when I either find out what they have to offer me or what I have to offer them, whichever comes last.
Certainly not ... first mention of possibility of aliance is in talk with Sazza, in Druid prison, so certainly not "walk up to a ton of very-likely-to-be-enemies" ... but much, much sooner. :-/
Also you can presume they would be "very-likely-to-be-enemies" as you want ... but you cant ignore the fact that no one in camp (yes the gate is different) dont attack you, until you start ... hells, they dont attack you even if you masscre their guards, and those guard during fight will use alarm drum. :-/
What is so "very-likely-to-be-enemies" about that?
Sazza is a simple scout who wants to be freed (making her an unreliable source) and offers a
way to get into the goblin camp. But I don't remember her saying anything about why I would want to go there other than learning about the Absolute, which in my view the typical evil main character has very little motivation to do. She knows nothing about my abilities nor what the Absolute's plan to attack the grove will yield (arguably nothing noteworthy) or why anyone would be really glad that I freed her (they rightfully don't).
About no one in the camp attacking you: again, you'll only find that out by putting yourself in the position that they could. A priori it is a high risk move with dubious reward. I usually entered the camp with Feather Fall from the North or by sneaking and then
entering the temple through the cracked wall.
By now I did a play-through where I freed Sazza and destroyed the grove. As many others here, I found it disappointing.
