2-The mountain had nothing of special at all ( I'm still wondering why Corypheus thought was good idea begin his mess there, I would rather pick the Brecilian Forest and a random dude who no one would miss), except being the tallest place near to Andraste's birthplace, the Guardian's immortality came either from an enchantment placed by Andraste or from the Ashes themselves
Wrong. It had some of the purest, strongest lyrium in possibly all of Thedas. Here's a quote of Oghren's, a dwarf who still has his lyrium senses.
"The lyrium veins in these walls are richer and purer than any I've sensed in a while. It's doing things... changing this temple and everything in it."
The entire mountain was effected by the lyrium, to the point of actually altering the very environment. This is also why the explosion that caused the breach was so destructive, well beyond what Solas anticipated. Lyrium is EXTREMELY volatile, if you recall. It can explode for basically no reason sometimes. So what happens when you take an explosion and add it to a mountain that is so lined with pure lyrium that the temple itself is being effected? What might have been a small explosion turned into a massive one because it caused a chain reaction with all the lyrium in the mountain blowing up along with it. This is also why red lyrium pops up there so easily afterward, the lyrium that actually managed to survive the explosion was very easy to corrupt. Solas didn't realize the Conclave was sitting on top of the largest lyrium bomb in Thedas, one that an explosion of moderate size could cause to blow up.