My word, you have done most of the work yourself. Well done.
The calls you have made should have been done by your IVA Company, that is what you pay them to do.
Cannot believe it has taken them 3 years to discuss the Student Loan repayments, and how can they justify sending you a breach letter now, by the sounds of it, you probably should have received that before.
But again, the error wasnt made by you, but the IVA Company.
Any offer that you wish to put forward should be done, the IP or their staff cannot tell you what you must offer.
It is your name on the IVA and you owe the creditors, it is the IP's responsibility to get the best return for you and the creditors. You can only offer them what you have available, and the chances are, with the current climate, the creditors may well accept your offer and get whatever they can. Something is better than nothing as they say.
Good luck with the refund and the offer that you request be put forward to the creditors.