@Amplify
Wait, isn't Colette supposed to be the main heroine? That's what they said in ch1. Anyway, you are right: the hero may end up with a sub-par healer, he may have some trauma by seeing a friend dying, then return home depressed instead of fighting the big bad, and the world ends. Sure. Or maybe the Big bad will come earlier due to MC introducing a agricultural revolution before time, or any of the changes he did. What's the point of having knowledge if he is too scared to use it? If he knows the story well, it wouldn't be so hard to make up some hypothetical and likely scenarios/deviations and prepare for them too. He is going to change things anyway, unless he wants to end up dead.
@Simpleton
No, someone from the hero party knows his true side, as he saved both her, her mum, and gave them a lot of money. As long as he doesn't worsen his relationship with them, I don't see why they wouldn't trust his fiance. Not to mention that if she is such a 'love even the evil monster' it shouldn't be that hard to make her go save the world.
To Both:
You both say that she won't understand that he is a tzundere, but every character he has interacted with so far has understood that. The likelihood of her stopping at his facade at this point is pretty low. He could make those who understand him explain things, if his mouth is not under his control. He could also create his own party using his knowledge, in case the hero has some problems, as it doesn't seem like he is weak. As per the parents, I'm sure some he can bullshit them by something akin to 'only our superior blood-like could save the world, not that puny peasant hero.' or similar.
Still, I get the meta-reason of giving him limitations, and it's a good story. Just saying that to me his strategy is not the best. Trying to make only the changes that won't change the future seems worse then make all changes you can while preparing for the unknown.