In my option framebuffer would be required only for nicer looking deinterlace – majority of other improvements could be done with just some additional line buffers and timing circuitry.
1. I wonder if that adapter implements framelock, because 240p<->480i switch changes refresh rate by around 0.1Hz. That is enough to desync most monitors no matter how fast a video converter follows the input, unless it operates at indenpendent refresh rate without framelock (also resulting to dropped/duplicated frames every now and then).
2. Doesn’t necessarily need framebuffer.
3. Doesn’t necessarily need framebuffer unless different deinterlace implementation is used.