They posed a query: which bottom; is it in relation to the oven - איבעיא להו תחתון האיך דגבי תנור
Summary:
ר"א may require crusting even the side facing the oven, while the חכמים are lax and crusting facing the fire is sufficient, or ר"א is lax requiring crusting merely facing the fire while the רבנן require entire crusting. The conclusion here that ר"א is stricter contradicts the ירושלמי where ר"א seems to be lax.
Summary:
We derive from the כם of מושבותיכם that אברים ופדרים דחול which was משלה בהן האור from ע"ש, may be burnt on שבת. The word מושבות teach us that we do not mete out מיתות בי"ד on שבת. The word מושבותיכם by שבת in פ' אמור teaches that שבת אי"צ קידוש בי"ד.