why this choice of not using the tigcclib.h-#include'd headers at all ? Out of necessity maybe ?

Because an update of tigcc broke an older kernel.h So I write it so that it can't be broken anymore by any update of tigcc.

That explains...
Do you by chance remember what change broke an older kernel.h ? Or, do you have some factual information (release, SCM) about when this happened approximately, which would help finding out what broke kernel.h ?

Most changes of kernel.h wrt TIGCC include files seem to be located in compat.h and kbd.h.
For the vast majority of headers #include'd by tigcclib.h, surrounding the "#include <default.h>" by #ifndef __KERNEL_H__ / #endif in the non-auto-generated part of most TIGCC header most files should cut most of the copy&paste while not interfering with the kernel.h definitions - and in the process, reducing the risks of missing several definitions and not keeping up with updates.


BTW, bug report: I noticed kernel.h of PreOS 1.0.5 lacks __Folder_Del:
#define __Folder_Del _rom_call(short,(const char*,short),66)