How do you keep place names (from smallest to largest) unit in the same "quantity"?
If I understand your question correctly, I don't think you can or should try to keep place names in the same "quantity".
Let's work this problem a little bit backwards. What should go in Place Details? Most people would put cemeteries and hospitals in Place Details, and I think also street addresses. Also, it turns out that if someone was born at home or died at home, then simply entering "home" without the quotes in the Place Details results in a default fact sentence that reads very nicely.
Then what should go in the place field? Most people would put everything else in the place field. As you point out, all of your "everything else's" will not have the same "quantity". For example, you might have "city, state" (quantity 2), "county, state" (quantity 2), "city, county, state" (quantity 3), or "state" (quantity 1). And some people would add "USA" or "United States" or "United States of America" at the end of each of the above, increasing each of the quantities by 1.
But to the main thrust of your question, it's very difficult to see how you could ever reconcile a place of the form "county, state" to a place of the form "state" to get them to have the same quantity when in the second case you don't know the county. There used to be a lot of people who would use a leading comma to indicate that the county is not known - something like ",state". That gets you to where "county, state" and ",state" each have the same quantity, but at the cost of huge ugliness, in my opinion. That is, I'm thinking of ",state" as having a quantity of 2 but the county is null or unknown.
I have finessed the issue of whether to include the word "County" or not when I have been talking about your quantities. That is, both "Sevier, Tennessee" and "Sevier County, Tennessee" mean the same thing and both are of quantity 2. I personally have a lot of heartburn over omitting the word "County", chiefly because places of the form "city, state" and "county, state" are of the same quantity so you can't tell them apart by their form. To me, the only way you can tell the two forms apart is if the word "County" (or "Parish" in Louisiana) is always included.
I have simplified all my examples by omitting any reference to Townships, Civil Districts, or other similar political subdivisions. I'm not sure if everyone agrees, but it seems to me that when these kinds of political subdivisions are used, they belong in the Place field rather than in the Place Details field.
I have not tried to address places outside the USA in this note. I think it would be better if researchers from those countries chimed in on this issue.
On the issue of including the word "County" (or "Parish") or not, irrespective of one's opinion, it doesn't change the discussion of your "quantity" because including "County" or not does not change the number of commas. On the issue of including "United States" at the end or not, it again doesn't change the discussion of your "quantity". Adding "United States" does require the addition of one comma, but you would be adding one comma everywhere. For example, "city, state, United States" and "county, state, United States" would both be of quantity 3, and they would have the same ambiguity factor as do "city, state" and "county, state".
There are a lot of legitimate differences of opinion on these issues, and I'm sure there will be additional comments.