This reply on reddit from a Bluestacks mod seems to also imply they don't support using the adb backup command (they seem to be trying to promote their own backup app).
I'm not sure if that means they've since broken it, or if that mod is wrong.
Regarding installing 32-bit, that isn't possible with the Bluestacks installer. It will automatically choose the version, if your device is 64-bit, it will install the 64-bit version.
If you try to use the 32-bit offline installer, it simply won't start and tell you to download the other version.