why is it that he couldnt go into sage mode alone? the reason he didnt against pein was simply that he had to remain still, if kabuto ressurects him, and is smart, then he will have him go sage mode before running into battle, granted in sage mode and with his willpower he would break the edo like itachi. If he couldnt break it, then at the least he would sacrifice himself to not harm the others by screwing with his chakra balance, turning himself into a stone frog.
He could go into Sage Mode alone. What Fukasaku and Shima were doing was allowing him to stay in his imperfect Sage Mode indefinitely by passing to him the natural energy they were gathering. Of course, it's highly unlikely that Kabuto even knows about Sage Mode, much less its weaknesses.