A couple of points to note. Andraste's mother, Brona, was a Ciriane, and must have been from an important family since her father, Elderath, married Brona to secure his alliance with a number of the small but wealthy tribes in the area that became known as the Fertile Crescent, which I assume covered roughly the area now known as the Heartlands in Orlais. Mafareth married Andraste for a similar reason and after she was taken prisoner by Tevinter slavers, he was obliged to negotiate her release in order to retain this alliance with the Ciriane tribes. So it is highly likely that the lands that were given to the elves were in fact lands that came to Maferath through his marriage to Andraste. Andraste herself may even have promised these to Shartan when forming their alliance because they were hers by right of inheritance and therefore she was entitled to give them to whomsoever she chose. Then Maferath, not wanting to reveal his treachery to Andraste, honoured the promise that she had already given to the elves. It seems highly likely that Andraste may have been aware of a historical connection with the ancient elves since the sword she gave to Shartan would appear to have been an ancient elven blade that originally belonged to Andraste's mother, Brona.
With regard to the treachery of the elves in not helping Orlais against the Blight. We only have the Chantry and Orlais' word for the fact that they didn't help. The story is that the Dalish just stood and watched as the darkspawn sacked Montsimmard. That is right out on the edge of the Dales, in fact I would say within Orlesian territory. So if the elves were watching, I think it far more likely that they had march there to help, only to find they had arrived too late to do anything useful, so chose instead to hold the line to prevent the darkspawn spreading eastwards. I repeat, we only have the word of their subsequent enemies that the elves did nothing. They could have been actively fighting darkspawn in their own territory. Most history that we read in the source books and in the codices is Chantry propaganda because it is based off what is generally acknowledged as history within Thedas itself. That does not necessarily mean it is the truth.
Also Orlais and Tevinter didn't exactly cover themselves with glory during the 4th Blight. Tevinter contributed absolutely nothing to the forces of Garahel and the Grey Wardens, apparently content to simply protect their own borders, whilst Orlais, largely untouched by the 4th Blight, did not send any official army, just individual Chevaliers who wished to help. When sending representatives to canvas support, the Grey Wardens specifically did not send Garahel or his sister because they were elves as Orlesians would consider that an insult. So much for solidarity against the Blight. It should also be noted that several places that Drakon liberated from the 2nd Blight had to win back their freedom from his empire subsequently. So the view of the leadership in the Dales at that time that Orlais was no better than Tevinter, was probably a pretty accurate assessment. If anything it was the 2nd Blight that kept hostilities from breaking out between Orlais and the Dales a lot sooner. Clearly there was already bad feeling about Orlais in the Dales in Ameridan's time, likely because they were worried about Drakon's intentions, but this did not erupt into violence until the end of the Blight allowed Drakon's son to look to expand their empire again, particularly since they had lost the Anderfels to independence even before the end of the 2nd Blight. The moment the Divine Hortensia named the new century the Glory Age it was pretty clear what would happen next.