Sure, the Noragami manga is completed. It was a great ride with an ending that wrapped up the plot nicely. If you haven't read it yet, it's worth giving a go.
Well, an omnibus in the world of comics is basically a single book that brings together a bunch of separate comic issues or volumes. It makes it convenient for readers to have a chunk of related content in one place. It can save you from having to hunt down individual issues.
A manga omnibus is basically a collection of several volumes of a manga series in one book. It's convenient if you want to have multiple parts of the story in one place.
It depends. If you're a die-hard fan and want to have a comprehensive collection of a series, they can be worth it. But if you're just casually interested, it might not be the best investment.
Basically, a comic omnibus gathers a series of related comic books together. It might include an entire story arc or a significant portion of a comic series. It saves you from having to hunt down individual issues.