Mike's site is a work in progress. For my blog I use his on sale dates for most of what I post, but last year I had to go through everything and change dates because he had changed on sale dates on his site. I found this especially with the Fawcett books. His on sale date revisions for Fawcett comics had them coming out much earlier than he had originally noted.
I haven't gone through the site this year to check if there are more updates--but I would guess these dates keep changing as more information comes to light.
Fawcett had a lot of intercontinuity between characters--moreso than many other publishers in the '40s. Stories like the introduction of Captain Nazi and Captain Marvel Jr. carry over from one title to the other. The Monster Society of Evil story continued for many months.
If you go to the indexes on Mike's Amazing World of DC Comics, you can look up entries for the three main Marvels (on Earth-S) and click appearances to follow the chronology that Mike has given for them. But keep in mind that this is part fact, part conjecture and part fantasy.
Mike is trying to incorporate stories from both Fawcett and DC in the way he imagines them fitting together in continuity.