Skeleton Key is basically to open the Master Bedroom without consent in the previous version. But now it is useless since you already have 2 Anna Routes to open the Master Bedroom without needing Skeleton Key anymore.
The Skeleton Key won't work on the Guest Room in Mario's Villa since you have to complete the alternate path of Sofia Route (Betray Sofia) in order to open it.
As for Safe Code 7172, go to Josef's Office on Friday and open his safe to get his gun which can be used when you meet a Debt Collector on every Monday at the Garage. Use the Gun to scare him ( or kill him?? ) instead of having to pay $5,000 each week.