I'LL TAKE BOTHHHHHHHHHHHHH
If I'm being realistic, based on my own characterization of the characters and how they live freely in my head; for prongsfoot to work:
1-Prongsfoot happened before Jily happened at all.
2-It happened during their later 30s-40s and after James had years divorced(also Sirius had 0 things to do with the divorce). (Meaning James and Lily don't die btw lol).
3-Lily is the only one who died.
4-Jilypad happened.
But I also think:
1- I don't personally like this option because this means no Harry.
2- Idk if, in this case, one of them would have caught feelings for the other before jily again (or Sirius during, cheating is a nono for James even emotional) , but I feel like due their age and many circumstances, they wouldn’t explore that, they cared about their friendship too much (like TOO much) I feel like that their bond was enough when they were in their teens and 20s. And I love the idea of jily falling out of love sometimes because it is realistic, and they do fall in love very young, and I love the idea of life crisis, trying to figure out shit, one having many patterns and roads, not meaning that one road is unimportant, but meaning that you go through it, change and flexibility and new emotions etc etc (this is one of my fav)
3-I love the angst of this, I really want to explore the idea myself. Harry being raised by James. Sirius not ending up in prison. Grief. A lot of grief. What would happen to the Marauders and the war, and especially Lily fucking Evans haunting the narrative etc etc.
4-Tops everything else. They're family<3
Even tho I have read cheating prongsfoot works, it's not like I mind them, but for me to read them, I go, "Oh, shit! I love the writing style, the wording, and the tension (if a work is good, I will read it. It doesn't matter if someone made James fucking blonde/now that I think about it blonde James is a nono, but you get me). But I have to disconnect with my Sirius, James, and Lily bcs I see these mfs loyal to a fault."