I would like to have footer nav and top level nav i.e. main menu, operating simultaneously on desktop and tablet. I want the footer nav setup to appear like one of the templates in the Legenda Theme Options. I have selected the one I like. But I don’t know how to activate it and implant our company’s sitemap etc. And when I go to WP Appearance >> Menus it seems I can only have one or the other operating on desktop/tablet i.e. a footer nav or a main menu nav, not both.
What you mean about footer menu?
You can select general menu + the same mobile menu in Appearance->Menu. See screenshot: http://prntscr.com/6xi8vo
If I understood you correctly, in footer area you can create menu manually via Static Blocks->Text block->adding code and then you can add the newly created block to Footer 1 or Footer 2 widgets (Appearance->Widgets)