It's easy simpler than you're making out to be the time machine Bulma made sent him to an alternate timeline a few years before his time. After he killed Freeza, he went straight back to his original, not having changed it at all. He then went back to the same timeline a few years later, when the androids showed up in his timeline. While there he trained, cell got killed, and he died and was resurrected. Then he went back to his own timeline again, still not having changed anything, but gotten stronger in the past. He went on to kill the androids and Cell, and the timeline continued until today.