1. No Pentium II system used DDR ram.
2. I also greatly contest your claim the USB booting is supported on old Pentium/K6/Pentium II or even Pentium III motherboards. Even socket 478 and Socket A motherboards rarely had support for that function.
3. Old motherboards have ram limitations. For example, the i810 and i815 series only supported 512MB total ram (2x256). The even older Ali Alladin V only supported 256MB ram. The 440BX allowed up to 768MB. You can't go shoving 512MB ram sticks in there all willy-nilly and expect it to just work.
1. Loads of pentium II systems used DDR RAM though most of them indeed used SD which coincidental even cheaper for those who are willing to find it.
2.The ability to boot from a usb device is not something depending on the chipset but rather on the firmware (BIOS) so again its not impossible.
3.Your right there are limitations to how many RAM and in what formation it could be used though these limitations rarely bring it under 256megs and thus sufficient ram is possible.
I do understand that your the glass half empty kind of person but that does not mean in any way that it is impossible.
Making your notes kind of redundant since any one knows there are system specific limitations to each system, this however was not the question the question was could it be done.