The one thing supporting staggering is that organizations with the worlds "United Earth" were in existence immediately after WWIII already. (That is, unless UESPA used to mean United Europe Space Probe Agency back when Friendship 1 was launched.)
So, we know that 2150 marked final unity. We know that there were several nations that only joined at that date and not earlier on, and Australia was one of them. And we sort of have the reason to believe that United Earth was formed in the 2060s if not earlier, perhaps with all of three nations joining and a fourth saying, "Er, umm, let's see".
We might narrow it down by listening to Picard and Q on the subject of "New United Nations": going strong in 2036, gone by 2079. "United Earth" would probably
not be a parallel, competing organization.
Possibly, but the Daedalus supposedly had a crew of over 200, and I doubt those older ships could hold that many.
Dunno. Archer's ship is as large as Kirk's, and Kirk's could hold 430. The older ENT types in turn are about as large as Archer's.
The fact that Archer only had 80 people aboard could be due to most of the crew being left ashore in the hurried departure of "Broken Bow" - and then all sorts of nonessentials being dumped for the Xindi hunt. The Essex
could have been a ship much like the old Intrepid
but, in the postwar environment, operating with a full research staff.