Question: Why does everyone argue over the best way to remove Cersei from power with minimal civilian casualties when they could have just sent Arya to assassinate Cersei? Given her training with the Faceless Men, she could easily infiltrate the Red Keep and get the job done. On top of that, Arya wants to kill Cersei.
Winterfell - S8-E1
Question: As Qyburn gives a crossbow to Bronn to kill Cersei's two brothers, he says "The queen's brothers made promises to you and broke them. Her Grace wants to rectify their mistake." I remember Jaime promised Bronn a castle or more, but I don't know what Tyrion had promised to Bronn and broke it. From I can tell, it was Bronn who turned his back to Tyrion. So, what was Tyrion's promise to Bronn that wasn't kept?
Answer: I believe Bronn had wanted Tyrion to give him High Garden, the Terrell's castle - which Tyrion was later unable to provide him. Jamie also offered Bronn a castle and also gave him a large amount of gold, but Bronn specifically wanted High Garden and would not consider any other as being grand enough. At the end of the series, High Garden has been given to Bronn.
Answer: Daenerys and her allies don't just want to kill Cersei, they want to claim King's Landing and free her people from Cersei's grip. The problem is the people of Essos viewed Daenerys as a liberator but the people of Westeros view her as an outsider and usurper. They would never follow Daenerys if she had Cersei assassinated. That is Daenerys' dilemma, she certainly has the ability to wipe Cersei out and obliterate her armies but doing so would make her a tyrant. Which as it turns out is exactly what happens.
BaconIsMyBFF
But no-one has to know that Cersei was assassinated. Arya has the ability to impersonate anyone she kills, so she could pretend to be Cersei afterwards and profess to the citizens of King's Landing that she has yielded the throne to Danaerys and that she is going into exile.
Phaneron ★
That plan would be incredibly suspicious. Knowing what they know of Cersei it is highly unlikely the people of King's Landing would believe that she would accept defeat so easily and then voluntarily exile herself, never to be heard from again. In order for that to work, all of Cersei's advisers and closest allies would have to be similarly eliminated, or they would have to be on board with the exile plan. If they are all killed it sort of makes it obvious that something is amiss. There's no way they would be fooled by Cersei suddenly doing a 180 and completely changing her personality by accepting defeat without a fight. If any part of this plan goes wrong then Daenerys would look worse than just an assassin, she would also be deceitful to the people she hopes will willingly accept her rule.
BaconIsMyBFF