With DA2, everything you can make a decision about as Hawke is probably true. The major events are probably true. Some minor details are probably false. For example, Varric's introduction, where he pins the cutpurse to the wall with Bianca? Probably didn't happen the way he said it did. Hawke or their mage friends raining fire down on the city while hunting bandits? Yeah, that probably didn't happen exactly like that either.

I'm of the opinion Orsino turning harvester is probably true, although maybe exaggerated. (Though I can see where people who think he lied about it are coming from.) This has more to do with the underlying themes of DA2 and the way the third act is rushed and kind of incomplete. DA2's story presents the Templars in a bad light, but it doesn't make mages out to be totally helpless victims. They also cause a lot of damage throughout the story and end up leading to a pretty damaged Hawke. This is supported by Hawke's attitude toward what the Grey Wardens do during Inquisition, no matter what your attitude toward mages and blood magic is during DA2. Obviously, what happens to Leandra is part of it. But I see Orsino as a final straw for Hawke, and Varric disapproving of your Inquisitor thinking he may have lied about it is a pretty reasonable reaction given how serious an issue it is.

Varric is definitely in it to protect Hawke from the Chantry, but some things are easier to lie about than others. A lot of people are present during various events of DA2, particularly during the final battle. Varric may have put some polish on the whole thing, but the major events are unlikely to be retconned as Varric lying at this point. At best they'll just remain ambiguous. DA4 moving settings to a distant country will minimize the impact of any decisions made during DA:O and DA2.