Ha back again and not facetiously (new favourite word)
I don't think at the time of Voldemorts downfall (in 1981) that Snape had worked out that Voldemort would survive. He seems fairly adamant on that point when he's talking to Dumbledore. I'm not saying he didn't work it out later but I doubt that. I'm also not saying he's not bright enough or that Regulus was cleverer but if he suspected horcruxes then surely he would have discussed his theory with Dumbledore. Regulus worked it out because of what Kreacher told him about V hiding the locket plus the Blacks were a family of Dark Magic and maybe horcruxes had been mentioned in that house. Snape wasn't at the DE reunion in Goblet of Fire when Voldemort talked about his 'immortality' in front of them.I have no doubt, he knew as did Regulus, Voldemort ensured some way of his not dying
By the time he officially joined Voldemort then, sure, he'd lost Lily's friendship but he was already well in with others who became Death Eaters, before then. He defends Mulciber's Dark Magic against Mary MacDonald as 'a laugh'. If Lily's accusation that he was already an aspiring Death Eater was false, at that time, then he could have denied it- but he didn't. To me it reads that he was already halfway down that path. I don't know how he imagined Lily would ever fit into that world, unless Voldemort's plans against Muggles weren't generally known to him but that seems unlikely. Perhaps he was secretly relieved that she stopped being his friend (contraversial moment)- just think of the strain of not being able to talk freely with her by his side all the time.By the time Snape joined Voldemort, he had already lost Lily's friendship and probably thought he had nothing else to lose.