Well, it doesn't seem that acpi_osi=\"!Windows 2012\" is helping. It seems that both this parameter and acpi_backlight=vendor expose the intel backlight interface, but we are unable to manipulate it. I wonder if the dual video cards are affecting this. I don't have any experience with dual video cards so I will have to step back (in case someone else can assist) and/or suggest you open a bug report for this issue.