Ah! I can't stop thinking about the mystery picture you posted~~ Is it a Daryun Serica special thing? And how did you find it? I, I... I need to know!! XD
Let me reveal it then, I don’t want to keep you in such suspense for too long!
You were pretty much correct - the character in the other images I posted is indeed a Serica side story-era Daryun, it’s a 20-page manga that covers part of the side story (not long enough to cover the plot in full and I don’t think there was ever intended to be a continuation, but I was very excited to get my hands on this anyway).
The manga artist is Natsuki Sumeragi, who I hadn’t heard of before, but I looked her up and apparently she’s well-known for her portrayals of Chinese history.
The manga is contained within a mook that features a section on Arslan Senki (other than the manga, it has a poster, character guide and battle logs and probably some other stuff, I haven’t had a chance to look at it in detail). The cover also features a colour illustration of Arslan by Shinobu Tanno: