Trunk dynamic stretches

[Caleb]

Hi,

First of all, I wanted to ask if the following sets/reps are proper for developing flexibility:
trunk:
rotations: 4x20 (20 = 10 to each side)
side bends: 4x20 (20 = 10 to each side)
forward bends: 3x15
bends to the back: 3x15


Secondly, It's a bit difficult to keep my torso upright when trying to sit like it is pictured in the book: with the feet in a V position while sitting down. It's also difficult to keep my legs immobile while keeping them straight as pictured, or trying to keep them on the floor while doing the side bends.

Is it ok to sit with the legs crossed or bent, as long as I don't move them while I perform the stretches? Also, is my inability to do so is a problem of flexibility, or strength (or both)?

Thank you.