Well, I suppose while he's still trying to fix the breach, the power imbalance isn't so pronounced, they seem like they are on relatively equal footing, and Solas doesn't do anything to dissuade her from that idea. You might even think he likes being thought of as some innocuous apostate hobo elf.
Then, for much of the game, she does have the power, and he has to depend on her to help him. And then in Trespasser, he reveals he is literally a several thousands year old demi-god who ruined the world - and is going to kill her and everyone she loves (possibly including even himself) to change it back. He has the power now, and she has to scrape at his feet and try to beg him not to do the thing.
The power balance thing is important, because it keeps one partner from taking advantage of the other, or from feeling resentful or used or even utterly dependent on them inside the relationship.
Edit: I'm not sure if this makes sense, because I am super tired, but I'm going to leave it up anyway. 
If we're going to put importance on power then I'll have to say I disagree there. I didn't see the ending of Trespasser as the power being at her having "to scrape at his feet and try to beg him not to do the thing". If anything, the Inquisitor becomes Solas' greatest weakness at that moment, and he hands it to them. Even to an Assquisitor who he doesn't explain absolutely everything to, he lets them live and gives them a general gist of what's going to happen.
Considering, if he wanted to upset this balance, he could have left with absolutely no one to know what he's doing to stop him. Even if the Inquisition's disbanded, the Inquisitor still has a lot of sway over people from reputation alone, they'll have informal contacts to call upon, and they're about to go to the one place where Solas doesn't know anyone to make infiltration a huge issue. So even though they lose the Anchor, and potentially the Inquisition, Lavellan still has a lot of power. IMO anyway. x)
I understand the whole issue in an IRL relationship or even in a DAI romance, it's just Solavellan never struck me as being "out of balance". They're both pretty powerful people, just in different ways.
I don't know if I'm making any sense either. x) It's 7:38am here and I haven't slept, so I won't be surprised if I come back later and all of this resembles Old God speech from World of Warcraft.