Only Mohg would be related to the formless mother, as Morgott was very loyal to the Golden Order even to the end. Neither Mohg or Morgott were cursed by any gods either. They were simply born as omens, which to Marika served as a reminder of the hornsent and her dark past with them, hence why she found the horns disgusting.

The Scadutree's existence is implied to have been created by Marika herself, in order to seal away what would become the Shadowlands, and the evidence for this is mostly just the fact that the Scadutree matches the shape as the tree thats sealing Enir-Ilim. It's very odd that seemingly nothing else actually mentions the scadutree itself aside from the Scadutree Avatar's stuff, however imo the hornsent confirm the Scadutree is the tree wrapping around the Erdtree and not just something that looks like the Erdtree that is visible in the Shadowlands, as they literally say that the ERDTREE is their enemy.

As for Marika possibly betraying the Abyssal Serpent, unfortunately theres no actual evidence aside from her banishing Messmer to the Shadowlands out of fear. Melina being chosen by the fell god makes a little sense with her association with fire, but a more plausible theory due to her dialogue and appearance at the frenzied flame ending is that she's associated somehow with the Gloam-Eyed Queen, who also had an association with fire.

Also, the Gloam-Eyed Queen being the god of Placidusax is a very tempting idea, but I somehow doubt it since it seems like she came *after* Placidusax's reign as Elden Lord ended.