If your client developed the design, why not ask for all of the development calculations and review them. This would be much quicker than re-design, but not as cavalier as just trusting their notes. Then you could stamp the drawings in good conscience.
A typical grout height for a 56" diameter stack would be 1 inch. The grout is intended to allow more exact trueing/ plumbing of the stack and more accurate placement at the design base elevation. As you get larger in diameter, and the potential variability of the top of concrete...
There is an article that addresses the non-linear nature of the guy wires. It considers not only the elastic stretch of the cable, but also the sag/span ratio and the initial tension:
A Design Procedure for Guyed Stacks Applying Non-Linear Cable Mechanics, by N. Gilbert, Y. M. Chokshi, Foster...