> But I still do not understand how one can consider writing to memory the OS owns to be ok.

Go to Vogons and look at all of the memory tricks people will use to get various games running on MS-DOS. This kind of juggling exactly which drivers to load, etc. is why Microsoft added the boot menu in MS-DOS 6.0 to CONFIG.SYS.

I'm not necessarily saying that this was the case here, but it smells like that to me.