That is not necessary in case of emergency fliegen_inlander. Although in most cases you would do so since rwy length is not an issue
Indeed you only use G+Y landing distance factor as this already includes the factors for direct law and flaps/slats jammed.
You only ever combine single failures and primary failures, not secondary failures.
Also you never multiply multiple factors with a * (ie flaps slats jammed and direct law), you simply pick the highest * factor and use that.