This code returns to the

This code will call CS:CS (instead of 0F000h:SubOff). Is that intentional?

procedure Call_Platform_bin_Sub (Sub_Off : Word); assembler;
asm
push cs
push offset @@return
pop ax
push cs {0F000h}
push Sub_Off
pop cx
retf
@@return:
nop
end;

The comment says {F000}, but CS is not F000 (unless uniflash is overwriting parts of the BIOS F000 segment with its own code).
CS is set by DOS when loading uniflash and can be any value (but not something in the Fxxx range). That means CS:CS will never point to any BIOS location.