We know that some algaecides, such as Polyquat, most certainly increase chlorine demand if you use a lot of it because chlorine breaks them down (it's not much in maintenance doses, but for high initial doses it's usually noticeable). We also know that stain removers also increase chlorine demand for the same reason, though EDTA-based ones break down much faster than HEDP. If you stop adding such products, then they will continue to break down and lower in concentration and the chlorine demand will drop. You should try that first; otherwise, you'll attribute the drop to the phosphate remover inappropriately.
I don't know why you didn't use 0-20-0 fertilizer I suggested for doing your experiment since that contains only orthophosphate. You'd need to add enough to overcome the phosphate remover you've already added. By using these other algaecide and metal sequestrant products you are mixing up chlorine demand from their breakdown along with release of orthophosphate (from HEDP). That's no way to do an experiment -- mixing multiple variables together both having affect on chlorine demand. Quite frankly, if you don't explicitly do the experiment using orthophosphate directly, then I consider your results invalid as far as concluding something specifically attributed to phosphates alone.