Sorry for your loss. No, your mother is not liable for the mortgage solely by virtue of being an heir. She may stand to lose the equity she might have inherited or been bequeathed if she doesn't step up and prevent the house from being foreclosed upon by keeping the mortgage current, but the bank isn't going to go after her simply because she's an heir.
I'm sorry for your loss and this tragedy.
I'm assuming you have an attorney and that you, personally, didn't take the driver's deposition, right? If so, you should be asking your attorney these questions and relying upon his/her advice. Don't go around your attorney and solicit case-specific advice on a general-information forum like this. No one on this forum can possibly know your case better than your attorney. And if you don't have an attorney for a serious case like this, get one...
You can file your suit in DeKalb; however, if the defendant responds with a motion to transfer venue to Cobb since that's where the accident happened, it will likely be granted. If that motion is not filed upfront and he simply contests the suit on grounds of liability or damages, or both, the case will stay in DeKalb. Good luck.
You really need to speak to an attorney about your specific situation; it's really not possible to give you competent guidance on a general forum like this. Use the Find A Lawyer tab on Avvo to look up a Probate attorney in Decatur/DeKalb County. Most will provide a free, initial consultation. Good luck.
No, a notary cannot validly notarize or authenticate his/her own signature.
However, the signatures of witnesses to a will need not be notarized for the will to be valid. There is a part of most wills called a "self-proving affidavit" which allows the will to be admitted to probate if the requirements of the statute on self-proving affidavits, including properly notarized signatures, are complied with. So, the will still is valid; it must, however, be proved up when offered into probate by...
It's impossible to answer your questions in this forum. No offense intended, but you're way out of your league proceeding (it appears) without counsel in fields of law (employment and federal tort claims) that are very tricky even when you have the benefit of experienced counsel.