Yes, this would be acceptable for an IVA, your IP will get you to do a budget and if this confirms that £720 is available to your creditors per month then that is what you would propose. I have one proviso though, H S B C should not be a significant creditor as they reject all IVA proposals with a dividend forecast of under 40 pence in the pound after costs, and yours would come in under that.
You would also be required to propose to revalue your house in the final year of the IVA and remortgage it up to 85% of its value at that time (at the moment it's mortgaged up to 83%), and the proceeds of the remortgage be paid in to the IVA too, however you don't have to do that if the amount that would be realised from this would come to less then £5,000, so this will only affect you if house prices rise during the IVA term (at present they're falling).
Now as you're both in debt you would both do IVAs linked together, this is described more in my page
www.freeivaadvice.co.uk/Joint_debts.htm .
You will have to demonstrate that the joint savings account really does belong solely to your mother-in-law. Do you have a paperwork trail that shows the initial funding for this account coming entirely from her own funds ? Is there any written agreement ro trust deed stating what the ownership of this money is ? If you can show that the money does belong solely to her then it will be safe from the IVA, but it may be prudent to make sure it is held at an institution that is not one of the IVA creditors.
