Here's my thoughts after reading you posts.
1. Cheatogro has a lot of phosphate in it. Maybe add iron instead. Potassium can help also. You can make a simple iron solution using Fergon tablets from the pharmacy. I'll assume nitrates are higher than 0. Your phosphate is high enough that if the Cheato isn't growing, it is a trace element or light deficiency. Besides helping the Cheato, iron can help with phosphate a little by causing it to precipitate out of solution.
2. If I had a big tank and a good skimmer, I would slowly drip a Lanthanum Chloride (Pool Phosphate Remover) solution into the skimmer neck. This causes the phosphate to precipitate out and be removed by the skimmer. I have used this method. It works great. You need to be careful though. Read up, ask questions, and be sure you understand the process before you start. If your rock had a lot of phosphate loosely bound to it, this process might take a while. It will take a while for the bound phosphate to leach out. It will still be way faster than any sort of phosphate remover like Chemi-pure or GFO... and waaaaaaay less expensive.
3. This is just a personal choice, but I don't put carbon in really high flow areas. I'm always afraid that the turbulence and friction will cause minute shards of carbon to escape the bag and irritate everything. It the cup you are using is super turbulent I would take the carbon out of it.
4. Water changes are a good idea.