Can I choose which leftover files to keep when uninstalling an app?
Short answer: Yes. Untick any leftover item on the confirmation screen to keep it; the space total updates. The app itself is always removed.
Yes. On the confirmation screen, each leftover item has a checkbox — untick anything you want to keep, such as settings you might want if you reinstall later. The total at the bottom updates to show how much will be freed.
The app itself is always included; that's the part being uninstalled.