I'd generally recommend having multiple saves in order to access all the scenes.I'm interested in knowing that too, I'm very close to the ambush thing but I think I have a lot left to do in Carceburg.
For example there is no returning once you've gone far enough down the peitho (Townhall) route.
I'm not entirely sure which ambush you are talking about, but in my current save where I am up to date on the main story I cannot return to Carceburg yet.
It sounds bad, but the game has a nice quality of life feature to counteract these dead ends. When you go to load game there is a "skip to content" button on the bottom of the screen that allows you to start at various key points throughout the game. This makes it relatively easy to go back for content you might have missed.