Actually, it doesn't matter if Williams lives or dies (other than missing the quest reward if he dies). If you've talked to him and triggered his decision to head downstairs, then you have the key you need to get into the mine. The next set of Black Ring soldiers shouldn't come upstairs until after you talk to Williams.
Did your father lead one of the first two Black Ring soldiers upstairs? That's the only way I can think that might work to 'let' Williams die without going through the conversation first.
Larian should be able to restore/fix that NPC with the save, if you email it to them at support@larian.com, with a description of the problem (DKS
support page). Unfortunately, they are on vacation until the end of the month.
The item you need from the mine isn't needed until you are ready to head to the Hall of Echoes, so if your father has anything left to do in the fjords, flying fortresses and Aleroth he can do that first.
In XP the saves are in the folder
C:\Documents and Settings\ %username% \Local Settings\Application Data\Divinity 2\Savegames
or in Vista / Win 7
C:\Users\ %username% \AppData\Local\Divinity 2\Savegames
You may need to set Windows Explorer to show hidden files and folders (
XP, Vista,
Win 7).
