Wow, this has been so helpful!
After sleeping on it and thinking about pretty much nothing else all day, I have tentatively decided to go back to my former married name, which is the name of my son, who is my very favorite person in the whole wide world! It's definitely the easiest option administratively, too. Also, I loved his paternal grandmother (my late former mother in law), and it kind of tickles me to be the next "Grandma Lastname," which is what he called her, and (presumably) have the same last name as any future grandchildren.
And... I am seriously thinking about dropping the second part of my two-part FIRST name, which I have hated every single day of my life (it's from the wrong era and the wrong part of the country and I've never felt like it suited me), and just be plain "Mary." That would give me pretty much the same reinventing-myself boost as a picked-out-of-the-air brand new last name, without the hassle!
Big fun!