You've opened the proverbial "can of worms" Aleš.<grin>
I know that the Wikitree standard is not to use any abbreviations, but I can't imagine there is a single user of Wikitree anywhere in the world that would not understand that "USA" means the same as the official name of the United States of America. Using this one abbreviation would save database space if nothing else.
"United States" is used on Family Search. But there is also the official name of Mexico (English translation) which is the "United Mexican States". I would avoid the shortened version (United States) as it is named in the Constitution and recognized by the United Nations by the full name.
I'd vote for USA.
As for a date, pick one! The constitution was adopted on 02 July 1776, but Jefferson's edits were adopted on 04 July 1776. Congress voted 09 September 1776 to name it officially. Each state ratified on different dates. Good luck with choosing a date.
And, yes, delete "Mass", it's wrong. And, officially, Massachusetts is actually the Commonwealth of Massachusetts, but that name is used only in official documents. Commonwealth also applies to Virginia, Kentucky, & Pennsylvania to be anal. But I doubt anyone wants to use that.
Perhaps for ease of use, 04 July 1776 should be used for the cutoff date?
Thanks for all you do! I, for one, thoroughly appreciate it.
[Edit] There is a list of dates here, though I won't vouch for its accuracy:
https://simple.wikipedia.org/wiki/List_of_U.S._states_by_date_of_statehood