There is no central list of shortcuts (keyboard accelerators) - they are visible in main menu.
Drawing tools accelerators are displayed in tooltips. Each group has one accelerator and they only work when the raster image editor is active. For example press B to cycle between tools in the Brush group.
There is no simple way to center your drawn shapes. If you want your shapes at exact coordinates, you can right-click a handle (the colored point you can draw) and the pick Modify handle coordinates form the menu. There you can type the exact numbers.