Ok if you have loose carbon in the vent valve you will need to replace the canister and blow the lines clear of any loose carbon. That should cure the P0455 if you don't have any rotted lines between the purge valve up front and the canister.
I doubt that is the cause of the miss but if the purge valve is being held open with carbon it could be, pinch off or plug the hose between the purge valve and the intake, the valve is on the left fender liner.
Still none of that accounts for a changing/weak spark. I have had to replace a couple of these computers for poor communication and strange readings when being scanned so I hope you don't have to go there. Without a scope it will be tough to see the signal to the coils and determine if it is poor signal or a bad coil. I was hoping you would tell me you have a Vantage pro or something similar.
You do open the fuel tank by removing the rear seat cushion, there is an access panel and the hoses are quick connects, be sure to cover the connection before you open it, I get fuel in my face if I forget this step. Keep the area clean so you don't cause a leak when you open the ring on the pump module.
Try cleaning the throttle body and see if that makes a difference, I would replace the crank sensor if the budget is not too tight, it is on the front of the block under the coils toward the front of the engine.
Let me know if any of that helps, and when you changed the wires were any of the coil towers corroded or full of white gunk?