As far as Prosa is concerned, the last time I played it I didnt speak to him at all until after defeating Serpent. The prisoner was in the dongeon in Belaine castle, he offered to give me the Moon rune if I would talk to the king about releasing him. The wierd thing about this is he gives you the moon rune right away. After this you can go talk to the king and pay 2000 gold to release him but as far as I can tell this does not seem to do anything since you already have the rune. I did speak to Prosa after killing Serpent. I am not sure if you can skip Prosa all together and go straight to Belaine after killing Serpent. It might be worth checking out. Could avoid going to Padal at all.
I don't know why the guide you mentioned said he was in Zellis. Perhaps this could be one of those things that changes depending on which character you are? I was Kamil when I did this.
You could test each of these alternatives and see which is faster.
Talk to Prosa, kill Serpent, talk to Prosa again to get Moon rune.
Kill Serpent, go directly to Belaine and get the the rune from the prisoner
Also, the prisoner thing is something I imagine most people who have played this game before have not seen so it might make the video a little more interesting.
I forgot to mention, and this might be important. I had stopped the war in Telaine and got the sky rune before doing all of this.