No, the playhouse is not required for the main plot. You need to get into the cellar in order to find the Order of the Viper Bracers (FoV warrior set), and to complete the play to get one of the pieces of jewellery you need for a minor side quest.
You can use the Developer Mode 'unlock' button to get into the cellar from the hatch on the opposite side of the street. For the jewellery, you can enter the playhouse (if you want to risk that lasting long enough to avoid a shutdown) and then use the 'goto' button (X = -12, Y = -3, Z = -3) to bypass the play and talk to the NPC you need to get the jewellery (then use 'gotocharacter' or 'goto Region' to get out).
If you wish you can email your latest save to me (raze@larian.com) and I can try it here. Make sure you have the jewellery quest from Ransid (sitting outside Chez Chanelle).
In Vista / Win 7 the saves are in the folder below. You may need to set Windows Explorer to show hidden files and folders (
Win 7).
C:\Users\ %username% \AppData\Local\Divinity 2\Savegames
