This question was actually asked - and at least partly answered - by an amateur historian named Elias Hillard in 1864.

He found at least 6 men who had fought in the Revolution and were alive during the Civil War. He photographed and interviewed them, and published it all in a book called The Last Men of the Revolution.

All 6 men were over 100, and you can read what Hillard wrote about each here. It's fairly dense prose, in a mid-19th century style, but fundamentally each is an interview in much the same fashion as you might find today.

This obviously isn't a full census, but I think it does answer the interview part of your question?

To answer your question, I downloaded microdata from the 1% sample of the 1860 and 1870 censuses from IPUMS (a website run by the University of Minnesota). I then dropped everyone who was born outside of the U.S. Since it was a 1% sample, I then multiplied everything by 100 to get an estimate for how many people were in the general population.

In 1860, there were about 230,000 U.S. born residents who were alive in 1787. By 1870, that number is only 68,000.

So what about those who were alive during the signing of the declaration of independence? Those are about 40,000 in 1860 and 7500 in 1870.

How many people would have remembered the signing of the declaration of independence and the end of the Civil War? Well about 1000 were age 10 during the declaration of independence and lived to see 1870.

If you are looking for interviews of people who witnessed both events you can consider the book The Last Men Of The Revolution was published in 1864, and contains photographs and interviews of 7 men claiming to be veterans of the Revolutionary war.

This page contains the interview from the book of Lemuel Cook who was the last surviving pensioner of the revolutionary war (he passed away in 1866), when asked about the civil war he stated "It is terrible, but terrible as it is the rebellion must be put down"

I'll leave the other parts of your question to others, but in terms of a census of the USA, that has been done on a decennial basis every ten years starting in 1790 by the United States Census Bureau. Overview of the statistics can be found here:

https://www.census.gov/history/www/through_the_decades/

While in the 1790 Census only the "heads of household" were actually recorded by name, subsequent censuses often went into detail as to even giving full names of every citizen in the country (within reason so far as the census taker was able to actually contact them) along with some pretty in depth demographic information for each citizen.

The 1860 Census had the data collected before the war started and in theory could give you a very detailed analysis of the information you are seeking including the actual names of people who lived that whole time span. Without pulling up the actual records and doing the search myself from the raw data, I can't tell you precisely how many people that actually was who were both U.S. citizens by right of having lived in the USA prior to the signing of the Constitution and were there the whole time. There certainly were some people though and even folks older than 74 in the 1860 Census. The question as to their age and even when they immigrated into the USA (if an immigrant) was on the census form for that year.

The following population demographics for the population of the USA in 1860 were as follows:

(Source: Population of The United States in 1860, published by the U.S. Census Bureau)

I hope that gets close to the numbers you are looking for on your question. I hope somebody else can show you at least an interview or two of those half million Americans that were alive at the time when the U.S. Civil War started that also were alive during the time of the American Revolution.

As far as I know, no historian has tackled this question directly. There were a good number of people who lived through the Revolution who survived into or through the Civil War, including some now-famous Continental Army veterans, but few historians of the time thought to question them about their opinions of the secession crisis. As such, modern historians don't have much to work with besides some snapshot examples. Take, for example, the account of Samuel Downing from the source above. In an interview published in 1864, Downing noted that during the Revolution " We had true men then; twasn't as it is now. Everybody was true: the tories we'd killed or driven to Canada." He added, a few sentences later, that he preferred the British to the Southern rebels because "When they was whipped they gave up. It isn't so now." This bit directly challenges one of the key components of the Lost Cause mythology that is only now starting to be dislodged from public memory: people at the time did not view the Confederates as admirable heroes for fighting on after they lost any real chance of a military victory.

But, again, examples like this are snapshots, and not broadly reliable. What can we look at, instead? The first example I would point to are the politics of many - if not most! - Continental Army veterans after the war and through the days of the early Republic. With notable exceptions (Daniel Shays springs to mind), former Continentals largely joined and supported the Federalist Party, which favored a strong central government and strong national identity. These men had served alongside people from other states, and had felt firsthand the consequences of a weak national government that could not and would not meet its obligations. Such people, if given the chance, would seem unlikely to support the Confederacy.

The generation of people born during or shortly after the Revolution also seemed to ultimately uphold the defense of the Union. The two examples I'm thinking of are Andrew Jackson (born 1767) and Winfield Scott (born 1786). As a military officer, Jackson was fiercely political, and openly served the interests of Southern planters (see his unauthorized conquest of Florida for a particularly glaring example). When some of those same planters challenged federal authority during the Nullification Crisis, however, Jackson responded sincerely and immediately with the threat of military force (though we can debate whether this was due to Jackson's defense of federal authority of him taking such disloyalty as a personal affront).

Like Robert E. Lee, Winfield Scott was a career soldier, Virginian, and a plantation owner. Unlike Lee, however, Scott kept his priorities in that order during the secession crisis. While Lee despaired about the prospect of drawing his sword against Virginia, Scott remained true to the army and the government it served, and laid out the plan that led to the Union's eventual victory in the Civil War.

Sources: Theodore Crackle, Mr. Jefferson's Army

Richard Kohn, *Eagle and Sword: The Federalists and the Creation of the Military Establishment *

William B. Skelton, . An American Profession of Arms: The Army Officer Corps, 1784-1861

Samuel J. Watson, *Jackson’s Sword: The Army Officer Corps on the American Frontier, 1818-1821 *