(God it is so annoying that I can't [/quote[ to break up these quote blocks. Anyone have any formatting tips?)
All things to find out in the game itself, but there's less than zero reason to write off a priori the possibility of siding with Briala. Especially when we might be able to get both her and Celene on the same side.
Not that debatable (and Orlais probably was attempting a sneaky form of conquest at the time, it's just that the Blight was worse and Loghain is a horrible politician), as Ferelden was still saved.
The Inquisition gets armies from other places, judging by all of the concept art thus far, and I'm not reading TME until I feel like giving myself a rage-induced coronary.
To your first point, agreed, though I would hope for an alliance with Gaspard. Celene is a treacherous ****** who can't be counted on.
To your second, ehh, maybe, but Celene doesn't seem like the conquering type. Besides, Ostagar would have gone a lot different if we had an extra three hundred Grey Wardens and a couple platoons of Chevaliers there to lend a hand.
To your third, every army counts. Tevinter is a viper than can't be counted on, the Free Marches are in disarray, Ferelden is recovering from the Blight, Antiva and Rivain are too small. I think Nevarra and Orlais are the only heavy-hitters left, and I wouldn't dismiss one so quickly.
As for TME, I'm not sure what you'd find objectionable in it. There's a clan of Dalish that are a bit more one-dimensional then I would like, but all of the other characters are really deep and complex, and the ending has an awesome twist that really shakes up the status quo, though I guess you've had it spoiled. I would definitely recommend it, especially to someone like you, who is interested in the plight of the elves.