When you're ready to add chlorine, expect it to drop very rapidly. I would add the dose needed to get to 15ppm FC and then brush vigorously with the filter set to recirculate so you can get the chlorine dispersed rapidly. Only go about 15mins at most and then test FC levels. Don't be surprised if your reading is somewhere near zero. With all the residual peroxide and remnants of biguanide in the water, the chlorine is going to be RAPIDLY destroyed (or, to be more specific, it's going to annihilate the peroxide and oxidize the heck out of the biguanide).
Keep adding chlorine until you start to get it to hold...you want to get it to 15ppm and MAINTAIN it at that level as consistently as you can. Once you start to see it hold, lets evaluate what kind of goop is in your pool. We could try to run it on filter mode OR you could try to let it settle and then vacuum to waste. Depends on how much goop forms and whether or not it floats or sinks.