I had a lot of fun writing Severus. Surprisingly. I was so worried whether I could do him justice, but then he showed up and took over, and all I had to do was write. ;)
Hmm, well, Severus hadn't planned any further than he wrote in the letter. He wasn't happy to hand himself over to Harry, so to speak, but thought him the lesser evil. But then, once he had re-aged, the Harry he'd got to know wasn't at all like the Potter boy he remembered. The new flash, on the other hand, was more or less planned. Severus discovered he likes being a teenager and with Harry. So he has come back. But now, while he's in a teenage body, much of his mind is adult Severus.
And one day, Severus will be bored of bein a teen and find a cure and stay an adult, and Harry will be happy to have an adult lover. :D