You can leave the palace before beating the boss. There is a switch that opens the main gate; it's near the entrance and it's marked by a symbol.
Also if you are still having issues with the boss and want to make her trivial, go to the cheat room at the beginning of the game and give yourself a ton of HP.
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Even if you reload the game there is nothing stopping you from getting to the switch with the red marking above it and flipping it as many times as you need to to open the main gate to the palace. I have done it countless times. I wouldn't just softlock the player like that, unless this is some completely new bug that I've never seen before.