Chasind are not a serious problem. They're peaceful now to the extent of some people actually fearing that Ferelden stopped considering them a threat which might end badly.
As for Ostagar's condition - have you seen this place? Rebuilding it would take some time and money, but as long as you want to have "just" a good fortress, not a Fort Drakon-grade keep, it seems quite promising. Tevinter was really building well.
So, basically, I disagree with your assessment, but I don't insist on Ostagar as long as it would be a fortress that could become something of an extraterritorial location without compromising Ferelden's strategic layout.
And as for Amaranthine, I understand that it would be weird to expect Wardens to partake in a national war - yet the Arl (regardless of how the position would be actually handled) would still be expected to do just that, especially since arls are generally in the "ok, now you're something like a bann with strategic stronghold entrusted to you" position - the main difference between arl and bann is his position in Fereldan military rather than their social position (though prestige of their position does make them stand out).
Basically, I don't know how it would work out in practice nor how would the vassal pledge look in this particular case. I'm pretty sure that there would need to be some clauses protecting Wardens from choosing between becoming traitors and going to fight Fereldan wars, but yet again - what if the enemy actually comes to their gates? Not sending Wardens into battle on the border is one thing, having them stand by and watch as Orlesians (or whoever) march into their (as in - ruled by Gray Wardens) city? If Ferelden manages to survive the invasion/win back its freedom, GW are likely to get kicked out again, regardless of what they contract said about fighting enemies of the king. Standing up to defend the city, on the other hand, creates a precedence of GW openly supporting some country against another in a war... Sure, it might be just this national branch of the organization, but still.
Basically, giving Amaranthine to GW should create a very problematic situation unless it was pretty much detached from Ferelden in the process, creating city-state of Amaranthine - and I highly doubt that Fereldan monarch would do that.
And as for the semi-independence, I don't think that's really the case. Basically, that's how Fereldan internal politics work - every noble is very independent. In Bannor there are constantly petty wars waged for wool, watering places or to intimidate a bunch of freeholders so that they understand that their bann is too weak to protect them so they should switch to supporting another one.
Frankly, I have a feeling that while Orlesian usual politics are pretty much a coup d'etait every Friday or so, Fereldan everyday politics imported to any other country would be called a constant civil war