Buh? The SNES doesn't have an "internal framebuffer". I don't know if it even has a linebuffer (byuu?).
I sincerely doubt it. The closest they have is a sort of pre-fetch line for OAM, to handle all the RTO stuff and sorting. That's why scanline 0 is black, OAM is preparing. And yes, modifying certain registers mid-frame will thusly throw off OAM rendering really bad.
Outside of that, yes. It almost certainly outputs one pixel at a time and moves on.