1. I would advise you to get a new solicitor to represent you at this hearing next week. The solicitor who is representing you has a clear conflict of interest between his own position and yours. He realises, which you don't, that you have a case in negligence against him if you loose this application. Accordingly, he is quoting you a prohibitively high amount of costs to go back into court to make your position clear and to defend this application. His own wish is that you don't go ahead as this will mean he won't be liable, which on the facts seems the most likely outcome. So you need to get alternative legal representation for this application. Basically, the solicitors on both sides have been negligent here and what both sides want is that the client(s) including you bear some of the losses incurred. So you need to resist that. Get an alternative solicitor to take up the file from this solicitor and give an undertaking as to costs to your existing solicitor. This new solicitor will have to put your old solicitor on notice that a claim might potentially be made against him should the application be lost. However, you would be very foolish to use your existing solicitor for this application. The other option is to represent yourself in this application, as you have done previously. Then, depending on the outcome, you will either have the 6k paid in costs or you will have a claim against your solicitor.
2. You need to realise that your existing solicitor is attempting to scaremonger you on the issue of costs so you don't go to court. This is because he bears the liability if you loose.