Can I return to the Overworld after entering the Underworld?
Yes, but ONLY until you touch the Orb of Life. Once you click on the Orb, you are locked in the Underworld. So, if you want to gather more eggs, level up more, complete sidequests (such as receiveing payment for delivering the wine to Ghedahre), or just explore more before the game ends, you must do that BEFORE touching the Orb of Life in the Memory Cave.