In step 6, khp was dissolved in a trivial amount of deionized h2o prior to titration with naoh. why was it acceptable to use an approximate volume of h2o when this solution was used to determine the exact amount of naoh that was neutralized?

Water acts as solvent and it has no effect on the amount of the acid or the titrating agent. 
Also, the concentration of the titrating agent must be known exactly, the amount of solvent is less important (the more diluted it is, the more accurate is the titration).