First of all you cannot be in default with your IVA if you only owe £600 and your monthly payments are between £300 and £400. Why is your IVA therefore likely to be failed? I suggest that you offer a repayment plan to your IP - perhaps an extra £20 per month until the arrears are cleared would be sensbible, or if this is not possible ask for the sum to be tagged onto the end of the IVA therefore extending the repayment period by a couple of months.
If you decide that bankruptcy proceedings are a better option, the Official Receiver is bound to allow you sufficient money to live a modest life - which to be frank may well be less than you are currently paying into your IVA.
With regard to the monies you have already paid in, by law the IP is entitled to draw down his nominee's fees in priority to payments to your creditors, so there is nothing stange about that. It is a shame, however, that you do not feel you are getting good service from this firm - who are you using?
Regards, Melanie Giles, Insolvency Practitioner