How has it occurred to no one that.... they can use their crazy skilled assassin who hates Cersei to kill her.

My $0.02...

Arya is pretty quiet and self-contained. She's obviously a skilled combatant, but I don't think she's disclosed her full skillset to anyone. I don't think anyone realizes she's a genuine assassin. Rather, they figure she just kind of learned how to protect herself.

Dany has been a little off the last few episodes. She's really got a personal grudge towards Cersei. She wants to be there to see Cersei die. Wants to personally remove her from the throne. I don't think she's thinking completely rationally right now. I'm not sure she's support sending an assassin instead of murdering Cersei's armies herself.

Jon is too honorable for his own good. I can't see him advocating for assassination.

I think Sansa might know that Arya is a genuine assassin - if Arya told anyone, it's her. And Sansa wouldn't have any qualms about sending an assassin after Cersei. But nobody is really listening to her these days.