Letter appear if you pop more then one monster at the same time, or if you pick up a white bracelet. When I started, I did not realize how long you actually need to wait between the levels (10.75 seconds) so I thought extend (takes around 12 until you can play again on the next level) and umbrellas weren't worth it. Though, now I agree that extend can save time, if used correctly. And about umbrellas, yes, I believe they will save time too, since you can skip that 10.75 seconds wait and the level, even though you may have to wait in the levels for some water bubble popping.
I made a 2 player run, but again, no umbrellas, cause I often clear the levels fast and few water bubbles appears, but same thing here, maybe it's worth waiting.
And about a complete 226 level run, well, I don't know if anyone wanna watch that much, cause that would take like an hour, and there's not very much difference between super bubble bobble and the normal one, only different colors and different monsters, but the monsters spawn in the same places. It's basically going to be the same run with some little changes due to different monsters.
http://www.mcboof.com/games/bb/
There's a good site with some useful info about bubble bobble, it seems to be about the arcade version, but it seems like things are the same in the NES version.
