I finished the book a few weeks ago, so I'm in the same boat. It's why I'm glad you replied so I could be reminded what happened in these first chapters!

I don't mind Locke's depression being shown in flashbacks. While getting to see him climb back from that without knowing the outcome would have been interesting, the story is so packed and even rushed in places already. There isn't time to include all of that, so I'm satisfied with the way it's done, because, as you said, it works well for pacing and framing. It also goes to show how utterly Jean is devoted to Locke, even when the latter is determined to be an idiot. That's really the theme of the book, how the two of them are devoted to one another to the point of self-sacrifice even when other things come between them.

It's a shame this is the only place we see the Bondsmagi in this one, but a nice reminder that they're out there, they will neither forgive nor forget, and no matter what victory Locke and Jean might secure in this novel, the magi aim to ruin them in the end, and can find them anywhere.

My only real complaint, though, is the in media res flash-forward opening. Yeah, it starts us off with a bang and shows that things will somehow get very bad for the two of them in the future, but we could have guessed that already. I found that having this scene at the beginning lessened my interest in it later on, when it comes up for real, word for word; and I didn't for a second believe that Jean was truly turning on Locke, even though I think that's what I was supposed to wonder about. If the theme of the book is how devoted they are to one another, then the opening is designed to throw you off and make you wonder all through the book how Jean could come to betray his friend. But it just didn't work for me. The very first thing I thought was that Jean was false-facing to trick their enemies, because that's what the Gentleman Bastards always do. So if I was supposed to be worried, then it failed, and I can't think of any other compelling reason to have that section as the opening.

However, I can't guarantee that I felt that way the first time through. It might only be hindsight speaking. And if I pretend that opening doesn't exist, the book opens quite smoothly and enjoyably, with Locke and Jean doing what they do best.