Hi there. First of all I would pop over to
www.iva.com and pick up the phone. Have a word with a couple of the companies there ( it's free and without obligation). You need to compare advice and see who you feel most comfortable with before embarking on a 5 / 6 year relationship with an IP.
As for the income problem. If you are living as a married couple they will expect you to be paying a fair share of the expenses (which is why they need to know what they are) and the proportion you are expected to pay will be proportional to your income. So if you earn twice as much as your pasrtner you will be assumed to be paying twice the share (2/3rds) of expenses.
Going on to the joint bank -- if your partner is solvent her credit references will now be affected by your DMP / IVA and her credit score will drop. This is because you are now financially linked. While in a DMP the damage might not be too bad, but once in an IVA it will be different. I would advise you to come off the joint account.
If any of your creditors are in the same group of companies as your bank you would be advised to change accounts anyway, to somewhere like the co-op.
Lots to think about, but better done if you chat to a couple of firms, as mentioned above. If you don't fancy talking to any over on iva.com, have a look around the forum as several post on here and you can get a flavour of how they operate.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014