Okay... I spent some time tidying things up in the tank and it's not AS bad as it looked. The cyano was mostly spread on top of the dying HC - it was very easy to vac off and I think if I stay on top of cleaning it up, it will stay gone. After vaccing and cleaning up the HC I noticed that some of it is actually still alive, so for now I'm going to leave it and see if it bounces back. I think the H2O2 may have been what killed it like that, not the lack of CO2. We'll see how it does.
I also put some of the driftwood in there - it's still a bit floaty so for now I tied it to rocks and will properly arrange it when it's fully waterlogged. I tied some peacock moss to it and hopefully that fills out nicely.
