Since posting, I've finished the game. I didn't have a save from the exact time I encountered the issue with Celia not being at the entrance, unfortunately, but I did load one from about an hour before that and played through just the Celia steps (which was not the way I was playing at the time, I was doing various characters) and this time it played just fine, with no glitched Celia. However, I did create a save file at the point when I had two Celia's in the same room from right after the game finally let me advance through Celia's plot (the double Celia did not happen to me again when I advanced through her plot again just now), so I uploaded that save to Google Drive if you want to take a look, maybe that could provide you some insight I hope.
Anyway, thanks again for trying to help. Here's the double Celia glitched save:
Google Drive
Also, I'm playing on PC for those wanting to know, with the patch and Lain's mod.