What is the earliest time for the first cup for the start of the seder?
I would definitely follow the Gra (Biur HaGra on SA OH 261:2; MB ad. loc., no. 23) here and start as early as 14 minutes after sunset.
The only reason not to start earlier than tzeit is the Terumat HaDeshen (137) – then paskened by the Shulchan Aruch OC 472:1 – who holds that just as matzah can only be at (full) […]