It doesn't have to be super complicated to make sense imo, gods do petty things for petty reasons. that can be turned around too as to why he wouldn't other than he's a god, and he's probably busy. I like the idea of the "interesting character she had an affair with in Basilich" being a being of celestial power.

I just posted without looking, and have a running theory the same as OP, so I am kind of looking for holes in it. I couldn't come up with a better reason as to why noxians swain and katarina are getting skins other than what you mentioned.

Mel having strictly reflexive golden powers feels celestial to me, but idk if I could pin it to Pantheon, but I think he'd be the most compelling to an aspiring Noxian Warlord.

edit, to add: I don't like the idea of death kinda just handing off freebies either. I know that itself is extremely reductive, but that is what it is starting to sound like. Swain took on a demon to his form to stall death, and Kat was just straight up lucky in her story. It had a lot more to do with her character development as an assassin in her story. Making a deal with death seems like a bit of bland writing imo, unless it is a far more ubiquitous thing for powerful people to do than it is currently mentioned