Hi -- you can allow your IVA to fail, at which point your creditors will add back lost interest and possibly late payment charges. Then there is a good chance that they will not agree to the DMP and pursue you for repayment through the courts, or secure the loans on your house, if you own it.
PPI payments will go into your IVA --- or the creditors will offset any refund against the outstanding debt, particularly if you are in a DMP. .
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014