Hi Al,

"7) If I then exit the high level routine after changing to
a new return address with an RETI exit, will it now unwind
correctly to the new return address? Absolutly, positively
......... YES."

NO ! NO ! NO ! NO ! NO ! NO ! NO ! NO !

Only after the 2. RETI the interrupt logic was also unwinded !
E.g.:
ext0_interrupt:
; ... do something
call iret
; 1. RETI
; now executing of high level interrupts possible again
iret: reti
; 2. RETI
; now executing of low level interrupts possible again


Peter


P.S. if you can not believe it, please try it !