Impossible to assemble as intended. The groove is too shallow for the slab to fit. It will protrude out and the hole for the screw will be misaligned (pic1&2). I managed to work around it and leaves a gap (pic3).
Moving on to next step, even after overcoming difficulties the space between the four legs are not enough to fit the shelf to align with the drilled holes (pic 4). I tried opening up the four legs a bit more but it is impossible to fir without potentially breaking the structure altogether.
Given above struggles, i dont even want to bother with the seat piece. I dont know what to do with this.