Read other letters about this article
In general, I agree with most of your points. Although it should be mentioned that they are mostly "gut feelings" that you (and I) think are what actually result from these encounters. There is no hard data to support those assertions.
On the other hand, I take issue with one of your statements:
"In addition, it tests the mettle of the candidate. If they can take the non-stop meet-n-greets, stupid questions, and constant distractions in stride - then it suggests that they can be level-headed in times of crises at the White House. You have to be able to keep it together."
These road trips by politicos into the "real world" (of America) are relatively easy to handle because of 2 things:
1) Most Americans have a general idea of how the rest of the country lives (as opposed to a Portuguese relating to how a Dutchman lives, for example). This makes it much easier for them to respond to questions with tact and generalities -- even if they don't really say anything relevant.
2) If a mistake is made it can be papered over with "clarifications" post-facto. It is not a situation where a mistake can cost you dearly.
The position of POTUS, on the other hand, doesn't allow you "do-overs" -- especially with regards to foreign policy actions. Once the decision was made to invade Iraq there could be no "do-over". If Israel were to launch a strike against Iran's nuclear facilities the POTUS will need to make an immediate -- and crucial -- decision: support them or stand down. There will be no "do-over".
So, you see, how a politician handles himself/herself in these situations does not indicate at all how well he/she would respond in a crisis situation for the USA.