Right now, there is no way to review all your previous choices. It would be super nice to have a way to view old levels when leveling up (or passing 100).
Even better, if each step had an indicator if you had taken that path. This would have to be based on the whole tree though.
It would also be nice if the past options showed which choice had been made at each junction somehow.
There is a way to view all your previous choices. Click/tap on your current Kao Chan on the dashboard.
Then select Collection. You'll see every one you've finished and the one in progress, and can choose which one you'd like to display.
That does not show up when you are in the state of evolving, which is what the topic meant. Which probably means you'll have to be aware of the issue before leveling/evolving up so you can save the screenshot whole level before or somehow recall it by memory.
You're correct, when making any choices, there is no preview available. This has been requested before, and I may add it at some point, although I can make no promises about timing.