On the topic of Solas leaving (both Lavellan and the Inquisition in general), I honestly suspect his motivations aren't super complex or even all that carefully considered. At least, not at the point of the story we're at with Solas when we see him last.
In the case of Solas ending the relationship with Lavellan, I think he pretty much did so out of what he judges to be necessity, after his knee-jerk reaction of fearfully shying away from the conversation he means to have. We know he's planning to tell her the actual truth when he takes her to the bone grove, but when it comes down to it, he gets cold feet and bails. At that point, I think he realizes his options are A.) Stay with Lavellan, keep quiet about the truth and abandon whatever it is he's doing (which he entertains for a moment), or B.) Stay true to himself, which means he must part from Lavellan, since he is too afraid to tell her the truth. Primary motivator: fear.
Uh, bye Lavellan! It's not that I've logically thought this out beforehand; it's that I've **** my pants because I'm so scared cuz' I love you so much and am terrified of the potential ramifications of my confession for so many reasons.
In the case of him leaving the Inquisition after Coryphetits' defeat, I think it was another plan B scenario for Solas (which the unfinished mural would suggest), in order for him to get the power he needed for whatever he's doing, since he wasn't expecting his orb to break. We can't know whether he's planning to return to the Inquisition or to Lavellan after the post-credits stinger, so I think it's going to be tricky to determine any motives for things that go beyond this.
TL;DR: My take is that Solas' primary motivator in leaving Lavellan is his fear of her reaction to the truth, and his departure from the Inquisition to go suck Mythal's power from her face was ad hoc.
Edit: I feel like I've maybe said this before, sorry if I'm parroting. It's meant in response to the whole "Solas leaving to protect Lavellan" thing, particularly. 