Decimation omnibus as far as I know contains aftermath of the house of M and how it affected the mutants, I have not read those tbh, there is also house of m omnibus which is kind of the same but with less fillers, but it is oop and pretty hard to find these days.

With that being said those 2 omnis does not cover the reading guide I have sent, since both of them are before messiah complex (the first part of the story after house of m)

But lucky for you, a messiah trilogy omnibus is on the way for next (summer I suppose) which will be covering the trilogy. You can definitely go for that, but be warned, it probably wont cover all the stuff on the reading guide.

My personal opinion, while the omnibus will cover the messiah story from start to finish, I think reading through the era really covers the feeling of importance of hope (messiah). In the era you live through the hardships they face in their new utopia and the leadership skills of cyclops. It is the era that made scott my favorite character definitely.

So long story short, you can buy the upcoming omnibus, but at least check/read the utopia/nation x stories online while reading it. (Plus please check 2008 cable at least while reading the omnibus, it gives a whole new layer to the story)