Maybe Snowy's walk to the mortal "end of time" takes so much of his time that, by the time he gets there, the Upstairs realm has advanced into its era when the demons regain their angelic status? No, because, as I understand the text, the fallen angels are still demons when Snowy has completed his return journey.
Either he has walked forward, then backward, along the first temporal axis and has endured through a short period of the second temporal axis or he has walked forward, then backward, along the first temporal axis and has endured through a long period of the second axis but has then endured backwards along that second axis. The latter would make life more complicated. In fact, he would be passing his outward bound self at every step of the way back.
It is not easy to think about more than one dimension of time (see here) and I may be getting this all wrong.