THC can be detected for weeks after use. So if it goes to trial it will be up to your attorney to inform jurors about it and convince them that you were not DUI. You should not smoke in your car because odor will me a police officer suspicious. Do not carry it in your car. Basically do not give police any reason to suspect that you smoke marijuana.
As to the cost it varies and depends what will be done. But you should expect to pay few thousand.
Most likely Nissan mechanics will be considered experts in the field. So their report will be given more weight than yours. I am not familiar with DCF in Florida, but you should think twice before involving governmental organization in such a matter. It may backfire. Discuss with your attorney first.
Many people keep everything they own. There ara rules which determine what you can keep. The debts on credit report will be set to zero. Most people in my experience have improved credit after bankruptcy. You will be able to by car, but rate will be higher.
You have to get a signed release stating that you are not liable for any known and unknown, current, and future medical and other expenses resulting from the accident from all people involved. You should ask for a couple of estimates. You can pay to the other party directly and have him/her sign a document stating that it is a total payment for the car repair, and that he/she releases you from any liability. You can pay the body shop directly, but you still have to get the liability waiver...
You should have responded to the lawsuit and raised the statute of limitations defense.
You should get the copy of your file from the court and consult with an attorney to determine if you can set aside default judgment.
There is not enough info to answer the question. You should consult a lawyer. If you cannot afford one, call bar associations and ask for list of pro bono legal clinics. Take all the paperwork with you and attorney from the clinic should be able to answer you questions.