It looks like the builder is still building, so why not ask him?

I'm confused - should there be another 2x4 at the bottom of the knee wall framing

That's not a knee wall, it's part of the truss.

That's not a knee wall. Those are trusses built with vertical members that mimic a knee wall and so allow for easily finished space. It's up to you to install whatever blocking is necessary.

I'd say the bottom chord of the trusses (the joists) are 2x8s on 24" centers. I suggest 2x6 blocking between them to support the edge of the rough flooring. Apply construction adhesive to the top of the blocking to prevent squeaks.

You'll need horizontal blocking at the top and bottom of the wall in order to have backing for the drywall and baseboard.

You'll have to fur out the angled area of the ceiling quite a bit to allow for ventilation and insulation.