Phil - one ASA researcher points out how badly worded surveys can give entirely false impressions. For example, one survey gave as one of their “origins position” choices something to the affect that “man was created specially within the last 10,000 years.”
It’s easy to assume that anyone answering that would be a YEC, but it would actually include Catholics who believe in an old earth but a specially created Adam, both mainstream Catholic teaching, and it would also include sophisticated Homo divinus views that are entirely conventional on biology, but believe the Adam story represents the conferring of imago dei or some covenantal spiritual role upon one or more humans in relatively recent history.
Another factor (which you hint at) is that those not committed to studying the matter (and quite a few of those who are) can easily hold contradictory views - one reason the choices given often add up to more than 100% of people. Many churches (certainly here in the UK) are just not bothered, so a Christian could happily accept Genesis as literal history, fossils as millions of years old and products of evolution, and never have even seriously considered whether they fit together.
To attempt an answer to Lisa’s first question, I think the problem is that nobody in real life comes to the matter of the age of the earth without some suppositions. Small children probably have no idea whether the earth is young, old or middle aged until they start having adult ideas instilled in them. Remember that ancient Babylonians dealt in at least hundred of thousands of years of civilisation; that many Greeks believed the earth was literally eternal, and that after that to mainstream Christians for 2000 years it looked pretty young - including scientists like Robert Boyle who considered Archbishop Ussher’s chronology an impeccable piece of science.
Nobody learns about potassium-argon dating before they’ve already had a decade or two of dinosaurs and “My First Astronomy Book” to make them accepting of the science, or alternatively implicit or overt Sunday School literalism and trips to replica Arks to be suspicious of orthodox science when they encounter it, and dig into Creation Science.