While Cernunnos is the most important god worshiped by the Fellowship of the Leaves, this religion also reveres Sucellus and Amathaon greatly. Amathon does not need another religion following him. Sucellus is now revered by all the gods as they see him as Nemed reborn, and his priests (the Druids) are already in the game.
Sirona's worshipers would be much more dedicated to helping the poor and the orphans rather than dealing with large scale political issues like diplomacy. Sirona's "guild" would probably provide health and lower the AC. I'm thinking this religious guild would be the Sisterhood of Sirona, as they are specifically mentioned in Perpentach's entry as the ones that ran the asylum where Perpentach grew up (that is, before he took control of every mind there). The Order of the Elohim greatly revere her too, but as that is a civ it probably shouldn't also be a guild. When first founded by Immanuel Logos, this group too was devoted mostly to caring for orphans. Diplomacy is more an Empyrean thing.
Actually, Diplomacy is mostly the role of Dagda. Kael has described the sphere of force/balance is very close to the sphere of Law, except that Law is about unilaterally enforcing standard laws and justice and keeping one's own oaths, while force is about making and keeping mutually acceptable agreements. Dagda's constellation event is the one that benefits diplomacy.
Although Ceridwen is awfully close to Agares, the fact that we know the name of her cult (the Emrys) and that it was actively involved in getting people to practice magic and convincing mages to join the Sheaim in their Armageddon rituals makes her seem like a good choice.
I'd say that Bhall is now at least as close to Agares as Ceridwen, as they even share a vault. I think Bhall is covered by the Ashen Veil.
I'm not sure Oghma should have a guild or many worshipers, as he wasn't even able to defend his vault and the worshipers whose souls had already moved on abiding there was being conquered by the forces of Mammon.
Arawn probably would be a bad choice, since his inaction discourages worship. Some (like the Sidar) hae tried to found cults following him, but his apathy didn't make it easy. They revere him, but thats not really worship.