D|S does have an undo function, in the Edit menu (or just press cmd/ctrl-z). It enables you to go back several steps, depending on how much memory you have, but doesn't persist between sessions. To export just part of a scene to Bryce, click the eye icon next to the things you don't want to include in the Scene palette (or cmd/ctrl-click to select them all, and then click the Visible button in the Parameters palette): invisible items aren't exported. Click the eye or the Visible button a second time to bring the items back.