Shadow of Light Dragon wrote...
So many Grey Wardens can leave the order (Alistair, Anders, Nathaniel) and no one is afraid they could spill secrets?
This is a significant one IMO. I've already said what I think about the Taint secret, but that certainly doesn't mean that someone couldn't spill the beans anyway. In DA:O we possibly could have countered this, as Alistair can be executed--heck, Riordan could even sneak out if Alistair *isn't* killed and finish him off with none being the wiser.
Awakenings, though, makes it clear Wardens can just up and walk out. Oversight? I dunno. I doubt all Grey Wardens who are conscripted hang around to Do Their Duty, so either there's a safeguard we're unaware of or other nations deal with deserters differently than Fereldens (who are majorly short).
How does the Right of Consciption work if the order is so secretive? You conscript someone against his will, force them to drink Darkspawn blood, why would they keep silent about the Joining?
I don't think the darkspawn blood is forced down their throats. I'd guess anyone who objects too strenuously is killed outright. Drinking under duress isn't a recipe for loyalty (though Riordan doesn't seem to think loyalty matters, IIRC...).
Well. Bioware tried to have it make sense. Sort of. Change things around or try to bend stuff until the pieces fit, it doesn't matter. 
I don't really see any of this as a problem plot-wise... allow me to explain.

First, it's been, what? 400 years since the last Blight? They Grey Warden numbers are significantly thinned by the time Origins starts, at least that's the impression I got. And not just their presence in Ferelden, but sort of dwindling overall.
So in Ferelden for the majority of ~20 years it's been pretty much just Duncan, though somewhere along the way - before Alistair's Joining - he picked up about 3 GW's, right? There are no details on whether these guys were imported from other areas (ie reassigned or wandered into Ferelden) or actual recruits of Duncan's, just that he had a few with him and they all died at Ostagar. I think you actually get to see them in the Dwarven noble origin, in fact.
Second, and forgive me but I cannot seem to recall exactly WHO said it but I think it was Riordin, there is an attitude that one is ALWAYS a Grey Warden, and even though you may "leave" the Order you really don't because it's in your blood, and all Grey Wardens will meet their end in the same place - the Deep Roads. So the Order may simply look at someone who says they're walking away and be like "yeah, whatever, see you in 30 years my brother" because you can't ever really leave the order, no matter what you say or think.
It's a part of you, period.
So in that sense, why should they care if someone wanders off or chooses to live elsewhere or whatever? In the Calling the two guys talked about how they had planned to live out their days in some remote area together, but since one of them died to the dragon, the other chose to stay in the Fade with the fantasy version of that dream. But clearly, they had planned on leaving the Order for seclusion until their respective callings.
I really don't think that it matters to the Order if a GW chooses to leave the fold, as it were, even with all the so-called secrets they like to keep.
Another thing of note is that as GW's age, there is this impression in the lore that they tend to isolate themselves from normal people, and maybe even seek out other GW's. This may have to do with some Taint-impulse, or simply depression, but if GW's tend to get hermity as they approach that age then there's another reason why the Order wouldn't worry about telling secrets... who would you tell them to?
Third, who cares if the "you might die at the Joining" is revealed? A GW who is asked the question would likely shrug, or play it off as just one of a hundred rumors about GW's. You KNOW there are plenty of rumors about the Order already, surely not all of them can be flattering "oh they're great warrior"-type rumors, can they? Some have to be negative and scary.
But then again, after 400 years and low recruitment in Ferelden, maybe the Joining IS a restored secret by sheer attrition of shorts. Who knows. But you also do NOT know for certain that the secrets Ferelden kept are also secrets throughout the rest of Thedas.
As for Anora knowing, well... what sort of ruler would she be if she didn't have the 411 on all the players in her political game, including the Grey Wardens? Does it matter how she knew about the Joining? I bet Empress Celene knows. I also bet that the First Enchanter knows, as does the Grand Cleric.
Do you think anybody reaches a position of power without learning how to uncover other people's and institution's secrets? I don't.
So, really, no big surprise there, either.