That's not bad at all, especially since we never found out for sure if it was Cersei who gave Ser Mandon the order to kill Tyrion (or did we?).
Also, welcome. /smile.png' class='bbc_emoticon' alt=':)' />
I don't recall for certain, but Tyrion assumed she was responsible, and I don't think we learned otherwise. Obviously I am quite prepared to be corrected.
that would be better than nothing, but if the chain already exists, you'd think that would have come up in tyrion and cerceis conversation about stannis. instead of cercei simply saying "we'll rain fire down on them" or what ever, she might have said, "we'll hold them at bay with the chain."
My solution definitely isn't perfect, but it's the best I could come up with. As others have said, the chain is more than just a means to victory in the story. It is essential IMO that Tyrion's cunning saves KL yet he receives no credit for it. And though I'm sure he'll keep his nose, I still expect him to be assaulted by one of "his" own men. I'm hoping he blames Cersei in the show as well, since they are getting along so well right now and that cannot last.