Even though the old agent was the major part of the problem, your issue is with the management company as you owe them the money. If the judge understands the detail of management companies, they will probably say to have to pay all the arrears although there is probably a very good chance that the judge will not understand management companies and rule in your favour. In that case the outstanding charge and legal costs will be shared by all the owners so you will still end up paying a little bit. If you do win you have to hope that all the other residents don't follow suit and look for equivalent refunds. The money will be coming out of one pocket to go in to the other with the addition of significant legal, agent, accounting costs.
If you lose the monies owed is unlikely to be completely deferred although the installments that you are required to pay may be very small given your circumstances. It would be useful to have a statement of your household means for the coming year so that if a judge looked for the money now or in larger installments you have the information to correct that immediately.