I would concur with hubble. No point in agreeing a deal that you are unable to follow through on. Personally, just me, not saying that this is the right thing to do, but one possible strategy would be to have your solicitor/PIP on standby, so that if they try to get a judgement against you, you go straight the Circuit Court for a Protective Certificate. This stops proceedings for 70 days while your PIP puts a formal proposal to them.
Also, try and stall them for a while. This is for two reasons. 1) Property prices are rising, which reduces your potential shortfall. 2) Tweaks to the insolvency legislation appear to be on the way which will remove the banks veto on insolvency deals and should improve your negotiating position.
Another possible strategy, not for everyone and a bit more nuclear is that let's say you agree to the deal, the 80% mortgage etc. Bizarrely, the existence of the mortgage may have the effect of protecting the family home in the event of you not being able to meet the terms of the deal, and go on to declare bankruptcy. As long as the mortgage payments are in line with the rental prices in the area, the Official Assignee will probably let you stay in the house, and will probably sell you his interest in due course.
Wishing you well.