If I may....
" how to have the login menu displayed on the off-canvas menu"
You can publish any module in mobile menu, just create/add new module and choose "offcanvas" position for it.
About changing +/- to "arrows" it request CSS override, the only method as I know. Try this:
.offcanvas-menu .offcanvas-inner .sp-module ul > li.menu-parent > a > .menu-toggler::after,
.offcanvas-menu .offcanvas-inner .sp-module ul > li.menu-parent > .menu-separator > .menu-toggler::after
{ content: "\f078";}
.offcanvas-menu .offcanvas-inner .sp-module ul > li.menu-parent.menu-parent-open > a > .menu-toggler::after
{content: "\f077";}