Even though they have offered, they can still add defaults as your iva isn't completed until you get your completion letter. Your debts are not written off until the end. You can then get them removed.
Sharing from experiences of dealing with debt
The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively.
Bob Marley. http://kallis3.blogs.iva.co.uk
As Jan says, defaults can be recorded even when in the IVA.
Personally I am going nowhere near my credit profile until my IVA is done and dusted! In fact, I never have looked at it - was always too scared beforehand.