5/8ths should not warp if installed correctly. Remember these are for covering over existing stairs. your end product is likely to be at least 11/8ths in thickness.
You can special order an 84 inch length x 11.5 depth x .625 thick retread through the store.
I think it could be a tripping hazard with the different height depending on how much. We used to have seats in a stadium and the step next to our aisle was slightly different SBS people tripped all the time. I’d replace them all abs stain them the same.
Yes if the tread is standard size.
The price is per piece.
You can but that is not mandatory.
Yes, exactly the color of stain I used. also put a coat of clear sealer on top.
I bought extra treads and cut off bullnose to put on landing.
Solid oak. Very nice!