The obvious reason for the substitution of the word "citizen" for "subject" is that there was no royal sovereign to be "subject" to.
Well Washington could have been one, but he refused to accept that role, but the word "citizen" was used in 1776 before they had officially decided what sort of government they were going to have.
This unusual and uncommon word for the time, did not appear
ex nihilo, it had a very prominent source for it's grand entrance into American usage, and this is the thing that other thinkers keep overlooking. The
normal word for that time period was "subject." Had the English Law thing been the source of their intent, they would have continued using that same word.
Your interpretation of that change as deliberately wiping out everything that applied to English "subjects" under the common law is directly contradicted by repeated citations by U.S. courts to English common law rights that by their own terms, applied to English subjects.
There is no question that there was legal inertia in the perceptions of the legal workers of the time. I don't think most Lawyers or Judges of the time gave much thought to the philosophy behind the creation of the nation. I think they simply regarded everything continuing pretty much the same as it was before, so there was no real need for them to correct or change their thinking for the everyday matters of law with which they ordinarily dealt.
I have examples of Lawyers, Judges and Lawmakers citing Vattel as the source of their understanding on all things relating to citizenship, and I have examples of others citing "English Common Law." What Judges and Lawyers subsequently say on the matter is not nearly so important as what Delegates to the Convention or Ratifications have to say about it. We have a pretty extensive modern history regarding the courts getting things completely wrong. (Abortion, Kelo, "Gay Marriage", etc.)
But on this topic, they had help in getting it wrong. I have personally came to the conclusion that one prominent and influential lawyer (William Rawle) was intentionally and deliberately misleading people about this issue. This issue is heavily bound up in the Slavery issue. English common law offers support for abolition, and Vattel does not. People needing to believe a thing, have a habit of deciding to believe that thing they need to believe.
A new word was needed precisely because a Representative Republic without a sovereign didn't exist. The concept of the two words is identical except for that one factor. So we were no longer subjects of a king, but citizens. That was the nature of the change.
So you are saying they completely overturned normal and universal usage of a commonly understood word,
(one written into virtually every legal document of the time) because of a compulsion to avoid a trivial inaccuracy? Like they were grammar nazis or something?
You overlook something. Where did they even get this idea that they had a right to leave the King? This idea certainly did not come to them from English common law. This idea to leave, and to use this new word "citizen" (which at the time meant "Townfolk" to English speaking peoples) to describe themselves, could not possibly have come from England. It was in fact Treasonous from the English law point of view.
But oddly enough, it was exactly what
this Swiss Republic philosopher of Natural law had suggested be done. So we have a clear path from Vattel to our form of government, and no path at all from English law to our form of government, yet we are to believe that all the characteristics of a "citizen" are meant to be the exact same as a "subject". Again, English Law of the time was hardly even cognizant of the word, yet it was the normal word when referring to members of the Swiss Republic,
and had been since 1370 when it is explicitly used in this manner in their "Priest's Charter" of 1370. (Founding Document)
Eytomology of the word says it's French, and that it's usage as "inhabitant of a country", traces to the late 14th century. (i.e. 1370, when the Swiss "city states" started using the word that way.)
Cases cited on the issues of bail, habeaus corpus, and all sorts of other rights that existed under the common law, and had been applied only to British subjects ,were routinely incorporated into American law despite the fact that we were all "citizens". And I'm unaware of a single case that actually turned on the distinctions between a "subject" and a "citizen".
I've already said the mundane aspects of English common law were continued. Madison said the same thing. It is those things about English law that were incompatible with the new foundational philosophy that were jettisoned, first among them is the right to leave. Expatriation was a concept not permitted to "subjects" back in those days.
So to toss out English law based on the change in that one word -- that again was simply reflecting the lack of a royal sovereign who ruled by right -- has no legal or historical support. It's just an argument invented 200+ years after the fact.
And yet that French word, and that Swiss usage of it have become adopted as our standard, while the English word and the English usage which were previously the norm, were not. Coincidence? Don't think so.
If you look at our historical documents, you will see
"Subject", "Subject",
"Subject", "Subject" used over and over again, with
"Citizen" not showing up in any degree until after 1760. We changed our word, yet you argue we had no intention of changing it's character? That we intended a distinction without a difference?
Why? It is pointless to do such a thing. If there is no distinction, then why would they not simply continue using the same word that had always been used?