Questions 1 & 2: Better to start over. There's no magic button that will convert old layouts to responsive ones. HTML5 uses different tags.
Question 5: Muse is consumer level software for hobbyists and non-coders who don't need a lot of functionality. It might be all you need. However, don't expect it do advanced stuff. You should plan to use Muse for the life of the site as the code it generates is not easy to work with in any other HTML editor.
Dreamweaver is pro-level software aimed at serious designers/developers. There's not much you can't do with it. You're only limited by your imagination and coding skills. Personally, I prefer DW.
Nancy O.
