Chrome omnibox: understanding differences between HistoryURL and HistoryQuick providers, and exact conditions for when a URL is suggested by each

Question

I am trying to understand the exact conditions for when a URL from one's history is displayed as a suggestion in the Google Chrome omnibox.

Here is what I understand so far:

• The AutocompleteProvider class gives suggestions to the omnibox.
• HistoryProvider is a subclass of AutocompleteProvider that specifically sends history suggestions.
• HistoryProvider has two non-test subclasses, HistoryQuickProvider and HistoryURLProvider.
• Both HistoryQuick and HistoryURL providers use the kLowQualityMatch constants (I am inferring this from the "If a URL doesn't meet any of these bars, it's de-indexed. Neither HQP nor HUP will return it." and "Just loosening them in RowQualifiesAsSignificant() will affect both HUP and HQP" in this bug report)
• HistoryURL seems to be older than HistoryQuick, and the former might be replaced entirely by the latter in the future (1, 2, 3)
• HistoryURL seems to have both a synchronous and asynchronous step (from here)
• HistoryQuick seems to only run synchronously (from here)

I have not tried digging into the code much, relying mainly on bug/mailing list postings and comments in the code.

My main questions are:

1. When does a URL from the history show up in the omnibox? Is it URLs visited in the last 3 days or visited at least 4 times or typed at least once? What are the rules for when a keyword matches?
2. Do HistoryURL and HistoryQuick differ in the URLs they return, or only in terms of speed and synchronicity? (They seem to differ in terms of URLs returned, based on playing around with chrome://omnibox/, and people seem to talk about scoring differences, e.g. here.) If they differ in the URLs returned, what is the difference?
3. What are the exceptions mentioned in "even those providers have exceptions" in this post (One possible exception I've found is when the session length is longer than three days, and one has visited the URL more than three days ago during the current session.)
4. How up-to-date is the material on https://www.chromium.org/omnibox-history-provider?

Disclosure: I posted this same question on the chromium-dev mailing list, but received no response.