Mid 90's FAA requirement was for FDRs to record 22 parameters. Leading edge device position OR selection and trailing edge flaps position OR selection are two of the 22.
Hard to know what the crew knew without the CVR. An asymmetry should lock that set of flaps (inboard or outboard) when the needles are two or three needles widths apart. But was the right inboard sensor damaged and stuck at the takeoff setting? That would lock the left inboards as they started to retract. Or did it generate non-existent flap positions that matched the selected flap setting? That would allow the left inboards to retract.
The best split/asymmetric flap checklist I've seen had pictures - if you see this (inboard and outboard gauges split), do this. If you see this (left and right needles diverging), do this.