The Gemara concludes that the Mishna begins with a discussion of Chazara, then proceeds to Shehiya, then concludes with a machlokes pertaining to Chazara – that Beis Shamai never allow Chazara altogether.  Why does the Mishna interject with Shehiya rather than first concluding with the din of Chazara and then proceeding over to Shehiya?

The Mishna states that fuel of straw doesn’t present a concern since no coals are formed.  If so how do we understand Bais Shamai  who prohibit (shehiya of tavshil or chazara) even by ‘garuf’ (coals were shoveled) when there are no coals present?