^Well though it's true Ikkaku's shikai can spin, it doesn't mean its reach is further than Hisagi's. If I recall from previous chapters Hisagi's shikai could spin and slice pretty far because of its long chain, whereas Ikkaku's shikai can't actually extend that much longer after splitting into three sections.
Was there mention of just two empty slots? I don't recall that, maybe someone else here knows? Though personally I don't think Renji is a captain yet - if he was I think somewhere down the line someone would have made a point to mention it, plus remember when Ichigo wanted to go into Hueco Mundo to chase Inoue, Byakuya and Kenpachi was sent to stop him. But he defied orders and went anyway, only for the other captains to arrive later. If he was a captain somehow I think he'd have been in on the plan. Besides, we haven't seen his haori yet...so we can't tell.