What the secret is, I won't say. Read through and learn by yourself.
Nearing the finish of the volume, what's left is a very short epilogue and even shorter afterword. Maybe I'll finish it tonight, maybe tomorrow.
For the "what characters appear" on OSO 5's cover, I'm going to make my own guess plus a small argumentation. Most likely it'll be Sei and Mikadzuchi, with Yun always being there. Why? Because Sei has less appearances so far than Myu, and Mikadzuchi not only had her design done, but also appeared in colour. Hell if I know who will be in front though. Possibly Mikadzuchi? I hope so, she's cool nee-san. Ultimately, this is my guess and there isn't anything in webnovel ahead that would suggest it. Pretty much any character can come up.
Well, then, the chapter link.
And, why is the first illustration where it is? I don't know, it was there in the book.