I'm back...Leebo alerted me to what you've been going through.
So there are two interferences here going on with the DPD test. You have high total oxidizer levels (peroxide + chlorine) and both will register on the FC side of the test. With combined chlorine there can be two interferences - if the CC levels are extraordinarily high, you can get CC breakthrough on the FC side of the test (as you learned about) and that will register as excess FC. Also, if you add the potassium iodide (R-0003) reagent to do the CC test, the excess peroxide with convert the iodide into iodine in proportion to the peroxide level, and you'll see HUGE CC numbers. One way to fix this problem is to dilute the sample. I would suggest diluting the pool water sample with distilled water in a 3:1 ratio (3 parts distilled water to 1 part pool water) and then measure a 10mL sample. You still use 0.5ppm/drop but then take your final answer and multiply by 4. If you have to, you can also use a 5mL water sample which is 1ppm/drop but again multiply by 4 if you use a diluted volume.
Leebo gave the right advice, use the OTO indicator to measure total chlorine for now. You're going to have to do some crude analysis with it and wait until you start seeing deeper yellow colors on that test before you can trust the DPD test. Right now the peroxide and chlorine are destroying one another and it takes multiple applications for the chlorine to neutralize all the peroxide and then, when the peroxide is all gone, the chlorine can start working on the biguanide.
It's encouraging to hear that you're not seeing a lot of color to the water. When only chlorine is used to destroy biguanide, you gets lots of yellow, green and brown colors from the chlorine oxidizing the biguanide polymer into smaller sub-units. High peroxide levels also destroy biguanides but with no color changes. So, perhaps, the peroxide did some initial degradation of the biguanide and maybe chlorine will have an easier time at it....time will tell....