I only ask because it's 2018 and I haven't seen any questions about this nor have I seen any content that proves the possibility to still shiny hunt those starters.
Yes, it is possible. You can soft reset for a shiny starter in USUM. They are not shiny locked. You can also breed to get a competitive shiny. Masuda method is faster than soft reset.
Yes. If you save before the Yungoos cutscene, you will be able to soft reset and the Pokémon may be shiny.