1st response: I guess so. Every translation I've read of it though, the ''eyes'' part were in quotations, and that can change things. I always saw it more metaphorically. I also assumed that if we're to assume that the Sage literally divided himself as Bodily and Spiritual energy and graced each son with the respective piece, that the ''eyes'' bequeathed to the elder son couldn't be ''complete''.. that the kekkei genkai/mutation/bodily-and-spirit required that gave the Sage the Rinnegan wasn't complete without the forces he gave to the youngest son. Are we then to also assume that the Uchiha Clan Ancestor couldn't somehow pass on the Rinnegan to his descendants? I still stand by my own theory, that the Sharingan is an ''incomplete'' Rinnegan. Where the Rinnegan can make virtually any jutsu.. the Sharingan can only copy virtually every jutsu. You may be right.. but still looking at the picture it doesn't stand out to me that the Uchiha Clan Ancestor actually had a Rinnegan.. I can see tomoe there. Also if we're taking this literally.. did the Senju Clan Ancestor physically take/implant the Rikudo Sennin's body?
2nd Response: I'm not arguing that if Kabuto summoned the SOSP, he likely wouldn't need Madara. I was referring to a case where you said Madara has no reason to fear the Fourth, to which I responded and posed a plausible argument against my own response, from which the comment of the SOSP was born.
As for the actual contents of the coffin, I'd be more willing to agree with you that something of Madara's is in the coffin, as opposed to his brother. All translations I've read thus far show Madara saying ''Where did you get that'' as opposed to ''Where did you get him/her''. Also, the Outer Path's Samsara of Heavenly Life technique, which returns the souls of the dead and revives those recently slain, was intended for use on Madara, and not on the inhabitants of Konohagakure. I find little reason to believe that Madara would actually fear his blinded brother, or even assuming his eyes were returned, a brother who is weaker than himself. I would say from your suggestions that the Madara ''soul-splitting'' scenario is much more probable.
Edit: Fun-fact - The coffin for what would be the Fourth Hokage never actually appeared in the manga. The Third simply assumed and negated the technique before it could be completed.