OK --- when you say 'joint agreement' you are both in your own IVA's but they are interlocking. Are you looking to finish the whole shebang, or just your part of it ?
If the whole thing I would think £2.5k would be looked at seriously and, yes, you can come back with an improved offer if it is rejected.
I have to say that, for peace of mind, the whole thing being closed early would be better, otherwise there will still be potential issues with the other arrangement affecting the property when it comes to equity release.
What happens if it is totally rejected depends on how your offer was couched ---- if you say "Our marriage has broken down and we wish to clear finances up and no longer be associated", then you can stipulate that, in the event of a rejection, the IVA will continue 'as is'. If you say " Our marriage has broken down and we can no longer afford to make payments", the IVA could be failed.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014