Changing cell validation on a protected sheet

I am trying to modify a cell's validation programatically on a protected sheet. Normally, this does not work since the interface and macros are both protected. A supposed solution is to use the worksheet.protect method on the sheet, and set the userinterfaceonly parameter to true.

This didn't work for me, so after some more inquiry, I found that I should also set the DrawingObjects parameter to False when protecting, since the drop-down menus used in validation are affected by this.

That sort of worked. The sheet was in a protected state and the macro successfully modified the validation as intended. BUT, despite being protected, locked cells can still be changed in the worksheet. So, it's not really protected.

I've attached a sample workbook. Any insight would be greatly appreciated.