Hi,
The Device Helpers interface naming is inconsistent for historical reason. There is no one base reference for the Device Helpers interface. If you are going to pick a reference doc to start with, I recommend pdd.ref from the DDK. For sources, the the DDK headers are the place to start.
You will find references that match "Device Helpers", DeviceHelp, DevHelp, DevHlp and a few others.
As you found, there are some references in the Toolkit sources, but they are far from complete and, TTBOMK, were never used to write device drivers.
Part of the potential confusion is that the Device Helpers are not API functions per se, but rather a set of request codes and data structures passed to a single function. The kernel passes a pointer to this function to the driver when it passes control a driver for the first time.
What you are finding with functions like DevHelp_RegisterPDD(char *PDD_Name, void *PDD_Function); are bindings defined in various header files that make it easier to call the DevHlp consistently for a given request. The bindings are available in various forms for various compilers and assemblers. From the DDK, there's devhlp.lib and the associated devhlp.h. There's also devhelp.h with a set of OpenWatcom macros.