I do have an independent medical opinion that agree with my assessment.

  You had stated here:  “I do have an independent medical opinion that agree with my assessment. ” A copy of that along with the death Certificate and autopsy should be referred to and attached to the formal Se4ction 1151 claim.   This is the actual SOL for FTCA claims in Tennessee: “Time Limits for Tennessee Malpractice Cases The victim in the case contracted hepatitis B from a colonoscopy in 2006. However, theVA did not inform him of the problems with the colonoscopy equipment until 2009.
The victim filed an administrative tort claim – the first step toward bringing a medical malpractice lawsuit against the VA – just 10 months after learning he had been infected with hepatitis. Unfortunately, by that time it was too late. His case was recently dismissed by a federal appeals court on the grounds that he waited too long to file suit. Even though the VA is a federal agency, medical malpractice claims against the VA are governed by state statutes of limitation. Tennessee has one of the shortest statutes of limitation in the country. Under Tennessee law, a medical malpractice claim must be filed within three years of the date the alleged negligent act occurred. It is important to note that this does not mean three years from the date the victim realized he was injured or three years from the date the victim realized his injury might have been caused by medical negligence.
The time limit is three years from the date of the negligent act, regardless of whether the victim draws the links or not. However, there is an exception in cases where the physician or medical facility fraudulently concealed the source of the injury. Because of this short time limit, individuals who suspect they may have been the victim of medical negligence should contact a Tennessee medical malpractice attorney as soon as possible. In many cases, filing a lawsuit can preserve victims’ rights while evidence is collected to support their claims .” Gatti, Keltner, Bienvenu & Montesi, PLC
This article is part of what happened to many veterans exposed to Hep B , Hep C, and HIV by the VA. “In 2009, the VA notified more than 10,000 veterans that they may have been exposed to hepatitis B, hepatitis C and HIV while undergoing colonoscopies at Veterans Affairs facilities. The VA said the exposure risk stemmed from improper cleaning of the tools used to perform the procedures.Approximately 6,000 of those veterans were treated at a VA clinic in Murfreesboro, Tennessee. The others were treated at clinics in Augusta, Georgia and Miami, Florida. To date, at least 90 veterans have tested positive for one of the three viruses. Unfortunately, a recent ruling in Tennessee has made it hard for exposed veterans to seek redress for injuries caused by medical negligence.” from https://www.gkbm.com/articles/court-dismisses-medical-malpractice-claim-against-tennessee-va/
You might still be within the 3 year Statute of Limits.Their are MANY malpractice lawyers in Tennessee. I think I posted links to some of them for you.They often have a chat feature and often do free initial consults  via email. Maybe it is too late.It would be up at 3 years after your husband died,as I understand this: “Under Tennessee law, a medical malpractice claim must be filed within three years of the date the alleged negligent act occurred.” The only redress for those veterans above harmed by the VA, who found out over 3 years years after VA had exposed them to Hep B,C ,and HIV, is Section 1151. I feel the lawyer you had did not understand FTCA at all.