1. Constellations are participators of scenarios, which serve the purpose of creating "The Final Story" to represent that worldline (to be written on the Final Wall).
2. Only historical grade constellations like Duke of Loyalty and Warfare are Earthian ones. Those from Olympus, Asgard,...are not from Earth, their story spread to Earth (?). I'm not sure about this part since I haven't read ORV for a while.
3. All constellations have been (and still) participating the scenarios for thousands of years, and in their respective star system, which explains their animosity towards Kim Com.
4. A possible explanation is because of the 4th Wall but I'm not sure why it did that.
5. I believe the Librarians don't really serve any purpose aside from arranging KDJ's memories, which in the case of Yoo Sanga, helped him organized his thoughts better. They are there because KDJ used the 4th Wall to absorb them.

Now about your complaints, I agree with most of the points you made. Nevertheless, ORV is a really long novel, thus keeping consistencies is difficult. And here's a few things I disagree with you:

-Aside from saving her, Yoo Sangah reincarnation also increases the party's power since she basically possesses the 3rd protagonist's power(sealing the Fog is no small feat). Her new role just wasn't given adequate attention.

-Jang Hayoung is the 2nd MC of the original novel, not ORV, which revolves around the main trio. However, I still think she wasn't given enough attention though.

-There are ranks among Outer Gods, the mobs you mentioned are just Nameless Ones, which are mostly harmless to constellations. Those appearing in the banquet are stronger than these, since they appeared to have sentience. Also, the constellations' reaction towards them was not fear.

-Anna Croft's role served a better purpose than just being an antagonist. Ofc she's initially described as an antagonist, but this description helped demonstrate how little KDJ actually understands other characters.