Thanks!
The biggest problem, to me is that by starting the conflict divided, you enable Bloodraven to defeat you in detail before you can consolidate your forces. Rebellions in the Reach and the Riverlands and then Bittersteel invading might have worked if the Iron Throne reacts slowly and defensively, giving you the space to organize and coordinate.
And with another Hand in charge, that might even have happened. But Bloodraven is far too aggressive for this scheme to work – he’s going to respond with overwhelming force and surprising speed (thanks in no small part to his superior intelligence network), and will crush any rebellion brutally enough so that it encouragez les autres.
If I was Bittersteel and had my prepared candidate in hand, I would have risked sailing around the coast of Dorne to land in the west. It puts a continent’s width between King’s Landing and me, so that I’ve got time to seize territory and gather together my supporters before royal armies can arrive. Add to that the fact that Dorne, the Stormlands, the Crownlands, and the Vale are all loyalists, whereas the Reach and the Westerlands and putatively the Iron Islands contain the bulk of former rebels.