level 10 cant be right. That would pretty much mean we got half the game already.
With how level gain scales that is a little inaccurate, but numerically correct on a shallow examination.
4) Level Cap: I read somewhere the plan is for it to be 10. Why? The story throws you into such high magic, high fantasy scenarios within the first 20 minutes and continues on that agenda throughout insofar as I could see; why limit yourselves at level 10? I feel like this story would be more well rounded out at a cap of 15, with the elements that are at play. This lets characters shine and also become appropriately powerful for the powers involved.
True true... I mean, the beginning alone isn't something a first level group should even be involved in. It seams like Larian wanted to tell their story no matter if it makes sense as far as adventure/level tiers go. It's what a very inexperienced DM would do. If anything, there should be some form of origin prologue spanning levels 1-5 that's much smaller in scale, before jumping into outright Planescape territory.
You are mostly right, although levels and what they mean have been scrunched in 5e; level 5 characters are essentially ideal for town-saving campaigns. A useful metric:
Levels 1-5: Gritty Fantasy (we did it, we saved the village!)
Levels 6-10: Heroic Fantasy (we did it, we saved the city!)
Levels 11-15: High Fantasy (we did it, we saved the kingdom!)
Levels 16-20: Superheroes (we did it, we saved the world!)