More "final" items with some "medium" imtes

I'm a mage and i'm playing vs Malzahar,or enemy team has warwick....i can buy QSS but ehn?? i don't need mercurial scimitar I'm adc and i want to stasis when fizz launch the fish..i can't!

and about zhonya..why i can QSS before finishng Mercurial Scimitar but i can't Stasis before finishing zhonya's hourglass?