FM AS i found very easy although it depends on what modules you take(and that depends on which syllabus you're on). one of my friends is holding an offer for economics at cambridge & he didn't do economics at alevel (he did Maths, FM, Geography & Physics alevel plus RS as level btw).
so believe me, economics alevel is far from essential & tbh i would just go with whichever one you think you'd enjoy more &, most importantly, which one you have time to study, coz doing extra alevels is extraordinarily time-consuming.
as for interviews. oxford & cambridge always interview unless your grades are low enough that they know immediately that they can't give you an offer. no other unis guarantee interview, they might be impressed enough with your personal statement & predicted grades that they give you an offer without interview. i was under the inpression that even the top ones like warwick only interview when they aren't sure about a candidate. (that isnt necessarily negative btw, it might be that they just have too many candidates who are all excellent & they need to do more than just read through personal statements in order to choose between them)