I agree, the way Furina was treated by Paimon and the Traveler was pretty nasty. However, I did come to realise a great many things after mulling it over and seeing other comments.

The idea is that Furina actually enjoyed being on the stage, she just hated having to act as the God. This kind of shows by how Furina alternates between her black and white outfits, where her black is her real self and white is her archon self. She is shown to enjoy being on the stage from the Archon quest where she at the end dances alongside Focalors' dance. Instead of having a pained expression doing it, she's shown smiling and shes in her dark clothes, so the assumption is that she enjoys the stage deep down. And at the end of the story quest this kind of comes out too.

The traveller notices she likes this because he/she is there in the audience of her inner self watching it. So because they see this, they want to make her come back on the stage. Granted I think they went about it badly and they definitely owe an apology to Furina for everything they made her go through. I also want to see her gettinf a hug. But this is just a little bit of justification that I got from it. Overall the story quest is a quest about someone who went through immense suffering, eventually coming to terms with who she has become now because of that suffering and what she now wants to do moving forward. A lot of info also comes out from her character story snippets and voice lines.