The Wardens are trouble. They're super soldiers with no enemy.
I would never let Wardens into my country. I know they're going to come running anyway when the Blight comes - since they can't control the urge. SO, until then - they can stay the hell out.
I find it suspicious that Wardens don't actually spend all their time cleaning out the Deep Roads. What ELSE should you be doing than killing Darkspawn... that IS your function.
Their dangerous philosophy leaves people at risk (and I won't even fly into another rage about what they did in the Fourth Blight) in the name of their cause.
After two more Blights - I hope the people of Thedas force the Wardens underground to finish off the remaining Darkspawn as they - and their group - are no longer needed.
That just means when Razikale comes, you are the one country that will definitely not survive. Unless the Blight starts on the other side of Thedas and doesn't get to you, but that would only be because the valiant Grey Wardens put it down in time.
And the Grey Wardens routinely send expeditions into the Deep Roads. There's way too many for them to kill, but they do try to close off passages and hamstring them in general.
And I don't know what you mean about their actions in the Fourth Blight.
There's a difference between "fortitude" strength and "powerful" strength. If I can endure all sorts of mental and physical barrages, that doesn't say squat about my ability to fight, because fighting is about more than just enduring.
Fortitude doesn't stop a sword from cleaving my skull, or an arrow from piercing my heart. And fortitude isn't related to muscles or a certain amount of magical power.
To be a good fighter, I need to understand something about fighting, and I need to be reasonably powerful (whether by muscles or magic). Surviving the Joining doesn't guarantee that, unless the Taint is slowed by sheer muscle mass. 
I don't really know what you mean. Fortitude strength=powerful strength. Enduring mental and physical barrages is a massive part of being able to fight, especially in Thedas.
Fortitude allows you to wear heavy armor for extended periods of time, and also helps you dodge quickly, which assists greatly when it comes to skull-cleaving and heart piercing. Fortitude is only related to muscles/conditioning. Are we talking about the ingame stat here?
The understanding about fighting can come later. If you are already quick, strong, brave, and perceptive, then you will be better than a person with training and none of those things. It's very likely sheer muscle mass slows down the Taint, in that big, healthy people have much stronger immune systems. It's not only that, you also have to be mentally stubborn enough to resist the Archdemon's call, but it's still a big part.