He needs the other eye for complete power over Rinnegan. He's been saying how he needs the other eye for like 3 or 4 chapters now.
The staff is probably incomplete because of either him missing the last bit of Kurama, his transformation being different than Obito's because of his body (like how DNA makes everyone look different), or just not having both Rinnegan.
Madara is can change his other eye back to EMS if he wants to. But he would never have to; the Rinnegan can perform all feats of the EMS. Secondly, he just needs to control the Shinju, and it's own Sharingan/Rinnegan eye will cast the Dream World.