about the items i prefer cleavers cauze the armor shreding and also the tankyness but you can change one of the cleavers to take death dance or spirit visage for the situation they are very good synergy with the rounes those items its one good late game build you can take wispers for forth item in the situation you need you may like take duskblade or youmous for the lethality and the cdr

about spells i prefer teleport and flash flash cauze you need to do flashy plays or escape as usuall and teleport for roam or back to lane remember its worth to spent them early

the pros of riven its she can do amazing stuffs on teamfights if you know well 'you can tankfights protect adc with your stuns[not always]good early game,she can takedown the carrys,fun to play,mobility
cons,its predictable,no big sustain,hard to master and need practice

she is rly strong she can engage on some fights but its not that your job ofc if you dont have tanks try and take some tanky items [you have the cleavers but maybe not enought]then take GA they will not focus you for sure you can takedown theircarrys from behind if you have good timing you can do a lot of things but also wacth what happent if they spent their cc ,ofc for riven you must split push and your team fed or farm you probably draw to your enemys so get ready to kill someone and if you can burn their spells also you can escape some times and your team will push but in order to teamfight you have to make sure you can fight you may need qqs or edge of night to counter their moves and most importantly stuns make sure you will zone ppl if you are tank or feeded that you will help your team find oportunity and ward around

make sure you will steal their jng if you can you will not stay behind you are not spending mana and you steal exp-gold from their jng so he must farm and most posible he cant deal with you if you are feeded ,and make sure you have vision for extra secure on escapes and killing peaple