I know that the ST-E2 does not have the functionality to alter the power level of slave flashes in manual mode. But can this be done via the camera's "External Speedlight control" menu. I've Googled for about two hours or more and come up with nothing concrete. Most of the posts concerning the ST-E2 were written before Canon included the in-camera Speedlight controls. Specifically, I want to control the power of an off camera 430EX II set to manual using the "External Speedlight control" menu of my D5 MkIII with a ST-E2 attached.
No, you can't. The ST-E2 is like a headless 550EX--it doesn't grok/speak the commands from the camera menu. You'd need a 580EXII or later hotshoe unit as master to have that kind of control.
If you don't have the cash to spring for the RT flash system or you want a small optical master, your best bet is probably to ditch the ST-E2, and get a 90EX instead. It's contemporary with your 5DMkIII, understands and can relay the flash menu commands, while being an optical wireless eTTL commander.
However, TTL/HSS radio triggers, like the Yongnuo YN-622C, Phottix Odin, etc. etc. might be better yet, since you won't have to deal with the range and line of sight issues of optical slaving, and will give you full manual and even Groups mode control (where you can mix TTL/ratios, manual settings, and strobe groups together).