This is the Colonial America (before 1781) project, so not the same as your question raised at https://www.geni.com/discussions/272643?msg=1689744. But I thought it an interesting place to discuss surname evolution in the USA and “legality.”
You had written:
I tried to find any documentation regarding how many surnames one could legally have in the US, and so far this is the best I have found:
https://www.reddit.com/r/namenerds/comments/12680na/can_my_us_born_...
It is definitely saying in US you can legally have two last names - separated by a space or a hyphen. Do you happen to have a reference for your statement that "American women had / have one surname at a time. That’s it. It’s the law of the land" Or do you view it as possible that "Smith Jones" is one legal surname, so not a contradiction to what you said (just as "Mary Lu" may be considered somebody's Legal First Name)?
I responded with a terse “yes” so as not to sidetrack the discussion.
I’ll start with a simple bullet point webpage from the US Government:
How to change your name and what government agencies to notify
You might change your name through marriage, divorce, or court. Update your new name with Social Security, the motor vehicle office, and other government agencies.
https://www.usa.gov/name-change#:~:text=file%20for%20divorce.-,Cour....