Compensating for hardness in aqueous NaOH solution?
The short answer to your question is that the dGH of your water is negligible compared to the amount of lye you are using, so no adjustment needs to be made on the basis of hardness. 15 dGH corresponds to 150 mg/L or 0.015% as CaO, which is trivial compared to your 2.5% lye solution.  

Other possible reasons for the slow cleaning process include the way in which the oil has polymerized (as a result of what has been cooked and under what conditions) and the quantity of polymerized oil present. Regardless, the means by which you can speed up the cleaning using your method are to increase temperature and increase lye concentration. If heating really is not an option for you, then you could try a 10% solution (as noted by Waylander in the comments). Disposal of this solution is not a problem, just dilute the solution first or during disposal by running water and pouring slowly. Many household cleaning products use lye so this really isn't an issue.