It's hard to say for sure. The availability of Venti in the story depends on multiple factors such as the story's genre, setting, and the author's intentions. It could be possible, but you'd have to look closely at the story's details to know for sure.
Yes, you can get Venti from the story. In the game's story, Venti is a key character. His story is intertwined with the lore of Mondstadt. For example, his role as the Anemo Archon is revealed through various quests. These quests show his connection to the city, its people, and the overall history of the region. He has a carefree yet mysterious personality, and his actions in the story often have a significant impact on the events that unfold in Mondstadt.
I'm not sure which game you're referring to specifically. Different games have different unlocking mechanisms. Some might require you to complete certain levels, side quests, or collect specific items. So, without knowing the game, it's hard to say how to unlock SSGSS Goku for the story mode.
You can unlock multiplayer after reaching a certain level or completing specific in-game tasks. But the exact requirements may vary depending on the version of the game.