If you start the game, then play it. Don't start the game and the go and do some random stuff. Simple. If you want to do some other random thing, then don't start the game or turn it off.

In ALL the games in existence, the opening cinematics have some amount of spoiler. Every single one of them. It is absolutely unavoidable to make cinematics for a game without adding a spoiler to it, though it is possible to make the spoiler less or more prevalent. Heck, this is the same with cartoons openings, so it's not game-related either.

And if you happen to end up on the cinematic?! Eh...turn it off?! If you don't want to be spoiled, you turn off the spoilers, right?! It takes what, one second?! In that one second you will not notice anything that would be a spoiler. There is a decently long introduction into what the song is, with nothing but a flag of Garlemard in the background. There is zero reason why anyone not wanting to be spoiled would be spoiled.

Though I agree it would be nice to be able to select the opening screen, there is no problem nor oversight to be talking about here at all.