You could perhaps do this: Crouchless% You must hold down at almost all times, with a few notable exceptions:

1. To open doors or any situation in a level that requires the use of the other buttons
2. On the world map
3. After you complete a level

If you want to torture yourself, go for it. I was thinking of doing it for NSMB2, but decided against it for now.

I think it shouldn't be allowed as well. As long as it uses a save state, I don't think it should be allowed. I'm not sure how a ban on that would be executed though...

For Any% optimal route, there are no more backup item houses on the world map. However, you can get an item house to appear at the start of the world by having the timer end in 11, 22, or 33, thus giving you another chance for the Blue Shell at the expense of some time (I don't know the exact time lost due to that). Otherwise, there is a good chance, I believe, that you'll encounter a Red Block or Hammer Bro sometime along your path, which can have a Blue Shell.

Just like Walrus said, levels are harder with the blue shell for the most part. Play it safe for a while, then incorporate the Blue Shell more into your strategy. I wish you good luck.