Yes, everything of interest will be dissolved in the DCM, only the performic acid is water-soluble. If you have added 70% of the performic acid already, I believe that you by now have converted about half of your isosafrole to the oxidation products.
Now simply separate the DCM layer and wash it with 2x100mL water, 2x100mL 10% bicarbonate and 1x100mL brine, then dry the DCM solution over MgSO4, filter, wash the filter cake with some dry DCM, distill off the DCM (recover for reuse) and subject the residue to vacuum distillation with a column, collecting the isosafrole (recover for reuse). The high-boiling residue (what's left in the distillation flask after no more isosafrole comes over) will consist of oxidation products, which you weigh and heat with a suitable amount of 15% H2SO4, then continue as usual to isolate the formed MDP2P.