AFAIK there is no way to just uninstall the patch; you'll need to uninstall the game (then reboot, make sure there is nothing left in the Divinity 2 folder, shut down all non-essential programs) and then reinstall the game to revert to an earlier version. However, saves made in the patched version may not be compatible with an earlier game version (don't know if there were any changes made to the save file format).
A couple people have had problem with the transition into the add-on content, though it sounds like you are getting a little bit past there. There are some suggestions in the following you could try.
Crash after Ygerna's memory cutscenesCan you start a new character at the beginning of FoV, and get past the point you have a problem with your existing saves? If a new character also has a problem, re-doing the Ygerna's memory fight probably wouldn't help (though may still be worth a shot if you can not get it working otherwise).
Try emailing support@larian.com, with a description of the problem and maybe a link to this post. See the DKS
support page.
In Win 7 the saves are in the folder below. You may need to set Windows Explorer to show hidden files and folders (see
here).
C:\Users\ %username% \AppData\Local\Divinity 2\Savegames