I like #29 and #27 as concepts. You've incorportated the vee model, and arrows to show process. #27 and #28 do not show the top return arrow to show the "validation" phase, and the gradation lines in the middle are upside down, since in systems design, the system goes from larger chunks to smaller chunks. In #29, I would like to see the bottom of the vee smaller, and the return arrow tip hit the left side of the vee.