Hi. I am afraid that your husband is already on the back foot by having to make up arrears, so the creditors would, possibly, be less forgiving this time around.
A Full and Final is usually based upon the number of payments still due x the monthly payment. It can sometimes be reduced if there are mitigating circumstances like ill health. However, in this case, I would think an amount close to that due would need to be offered.
The IP is not being awkward in not getting involved with the formulation of an offer, it is simply beyond his remit as you are supposed to make your "best offer". However, he is being awkward in suggesting it is a one off, as you could come back with a better offer (bearing in mind how this would look to the creditors to discover the first offer was not actually the best!).
Bankruptcy will only follow if that is the way your proposal has been written, however, in view of the fact that nothing would be forthcoming from bankruptcy (other than, possibly, three years payments) this could also be addressed in the offer, but with no guarantee of acceptance.
I have, in the past, seen very low offers accepted, but, on the other hand, I have seen (although rarer) offers of the full amount and more refused. It is literally in the lap of the creditors.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014