Just finished the book.
Its quite a good read, but still have some queries...
I had some real trouble understanding some of the story

.
SPOILERS ahead!
(Where have the spoiler tags gone?)
1. How is it possible for a Marine chapter to have such low discipline? Squabbling in public with lowly mortals around? A very bad, shameful look!
Those mortals are their own servants. Would YOU care if your launderers were around when you were having a tussle with your brother?
2. The Emperor as the "God" emperor. I know in the old background Space Marines did not see the Emperor as a god and were looking down on
mortals who do...since when has this changed? And shouldnt there be a common ground in one chapter after they all get the same indoctrination?
There is no single view on this. Some chapters consider him a god, some don't, some fall in the middle.
3. The defense of the cemetery world- is it really believable that the Marines are going to defend this pretty useless rock? Or did come into account
that the Excoriators are Imperial Fists successors?
Its not useless, or did you miss the thrust of the scene with the Ecclesiarchy?
4. The support and understanding for the civilians- now after reading "Helsreach" I came to think this is very unusual for marines. Weren't the Salamanders
one of the only chapters close to normal mortals?
They are noted as being more humane, more than is usual/normal for Space Marines. Nothing in the lore suggest that they are the only ones who can act like that.
5. At the feast of blades- wasn't the Fire Lord cheating with his fire breath? I mean, the chaplain has to switch his rosarius off and nobody is allowed
to have more than a sword- and then the Fire Lord has his own flame thrower?
I suppose.Not that it really matters though. Swords were still prominent in the fight.
6. The tail of the comet- why were there many different demons there and not just Khornate ones? If the comet is attracting Khorne worshippers
because it has transformed into a Khornate omen in the Eye- where do the other demons come from?
Shrug. Been a while since I read it, but they could simply be the detritus of the comet's manifestation in the warp. The warp isn't made of JUST the aligned gods after all.
7. The Dorns Blade. Did Kersh steal it? For what reason? And why didn' he get punished for that?
Santiarch explains all of it. And are you really going to punish a captain who just defended a vital Ecclesiarchy world and has become a hero to the world's people?
8. Kersh was made Chapter Master.

After all the drama about everybody hating him, he finally gets respected enough to lead the chapter?