I don’t think the way the Salary Structure and Assignments are architected, you can do any of the things you guys have posted in this thread. In a lot of cases organizations let their systems dictate their processes rather than let their processes dictate their systems.
In this particular case, if you want ERPNext to be modified to suit your processes you have to wait or you have to sponsor the changes. So, it’s better for you to have an HR Process that says that Salaries will only be changed from the next Salary Period. You could perhaps pass on the 15 days salary as an additional Salary for the previous month.
For the 10% increment, you have to do a new Salary Structure Assignment and change the base by 10%. If you have hard coded all of these in the Salary Structure, your easiest (or at least - least hard) way is to download the existing Salary Structures in Excel increase by 10% and upload the Salary Structure and reassign the new Salary Structures to employees.
I know it’s not an elegant solution, and unless you are willing to do customizations or prevail upon somebody in the community to develop these enhancements, I don’t think you can do much else.
Hope this helps.