the seller can't even sell house if there is a lawsuit filed

I think this is the most powerful weapon, if the seller can not sell the house and has bought another house, he will have big financial burden. Find out his financial situation first and local reputation.

it is better to seek professional advice.

you may lose your deposit, but the seller may suffer more.

Also do not forget to negotiate with the seller in a calm, frank and firm manner. Do not get angry, business is business. You can be mean in business but do not let emotion get in the way.

remember also to fire your current attorney if you suspect that he/she is not loyal, you do not need specific reason to fire him/her.