"all FIVE of you are knight apprentice"... sus.

Likely because the author is trying to portray this story as something more grounded in reality and realistic. If someone faced their deaths only to be reincarnated on familiar surroundings, it is likely that they'd attribute it to a dream, almost as a cope mechanism. The only reason as to why stories nowadays forgo this and go straight to the action is either because the story they're trying to tell isn't trying to be grounded in reality, or because they expect their audience to know it already, and that they do not want to read the "prologue" again.
The same applies to his personality, while you see a lot of people brag as they imagine themselves to be the first ones to rise to the occasions on situations such as these, history tells us otherwise, they get scared, even when they don't want to be, it is the reason why we remember the people that do, as there are a ton of people who could've but do not. It also opens the character to receive character development, as they start "just like us" and as the story progresses he becomes a "true protagonist" per say.

It is likely just one artist. It is very common for new artists to learn how to draw by drawing realism, they do this so that they master anatomy and become able to deconstruct it at their will, allowing them to stylize their drawings while still maintaining the laws of the anatomy.That all to say that is is very likely that the author can just draw in both styles. As to why have two styles, well manga is generally a weekly release or monthly, and those drawings tend to take about... 5-10x more time to do than the other one.