A: Yes, using the Universal Pokémon Randomizer after patching, but it may break some intended encounters.
A: Yes. Every Pokémon is available in the wild, including starters, fossils, and version exclusives. Renegade Platinum Wiki
A: Yes – Gym Leader rematches, Elite Four rematches with full legendary teams, and the Sinjoh Ruins event (Lugia/Ho-Oh/Dialga/Palkia/Giratina). A: Yes, using the Universal Pokémon Randomizer after