The plan to capture a wight is stupid
Risking their lives to convince Cersei to cease fire and collaborate until the war with the undead is over seems incredibly stupid and unnecessary. Especially since they can clearly take out Cersei right now as long as they can deal with the fact that there will be a lot of collateral damage.
The smarter plan would be to recruit Arya (and her magic faces) for a clandestine mission to infiltrate KL and kidnap or kill Cersei. Or better still, maybe even kill Cersei and take her face/identity. As soon as she is deposed, the war is basically over since there would no longer be a Lannister heir to the throne. Everyone would just have to get on board with Dany and fight the walking dead.
I'd make a better Queen's Hand.