NO. School leaving certificate is not mandatory while confirmation of application at Facilitation Center (FC).
Ref. Annexure-II in information brochure https://dsd19.dtemaharashtra.org/dsd19/admin/uploads/Diploma_2019-20_Information%20Brochure%20(1).pdf
For open category: 400 & all other: 300
No, report to institute
Yes, before display of provisional merit list through FC.
You can add maximum 300 choices.
YES, if you have been allotted seat other than first preference.