I will think about it, thanks!

Overall the seat looks very excessive. Is this really needed? Wouldn't be two cross walls just enough? Especially when comparing with the other model. If you want more cross stability just add back the material of the arc cut-out at the bottom of the two remaining boards.

Does the back rest (which is short) really need to be supported by ten pieces?

And what do these legs in the middle do? Transfer force up that has been transferred over by bottom cross beam? In the hope that this connection will be stiffer than the legs going up directly?

Putting the same amount of material onto the back of the other legs would at least give additional strength to side forces and might prevent the only possible mode of failure: buckling to the side.

The size of the player required al lot of reinforcements, this is the reason of all this material! But i will work on your suggestion, thanks!

Oh, an even easier, less intrusive improvement would be putting the middle legs at an angle. So they meet the outer ones at the casters and the touch at the top in the middle of the backrest. That way they can directly receive the force from the casters and give stiffness to cross forces due to the triangles formed between the four legs. 

There is a lot of material in the back corners that doesn't do anything useful. I first thought it might be required by regulations but that wouldn't make much sense for badminton.

May be a thin slat connecting the back casters to roughly the axle of the main wheels could do the same job.