Constitution - From the suggested list this name does appear onscreen, but in a very obscure way. The name appears on a phaser schematic seen on a viewscreen, but was completely unreadable until the schematic was reproduced in magazines years later. Even on Blu-Ray and enhanced I'm not sure the schematic is legible. As such it can be debated whether Constitution is indeed an Enterprise equivalent in terms of canon.
Well, no, not really. To have a
Constitution class (which "Space Seed" and "Trouble with Tribbles" do show), you need an actual
Constitution. Otherwise the class name means nothing.
Not necessarily. The Constitution could have just been a testbed ship, much like the Nasa shuttle Enterprise.
But, I don't disagree with your reasoning. I think unless you take apocrypha into account, you can only count what we saw depicted as Constitution Class ships on screen, as confirmed Constitution Class ships.
By my count, that would be:
Enterprise from TOS and Movies:
Enterprise 1701
Enterprise 1701-A
Mirror Enterprise ISS-1701
Other than Enterprise:
Constellation(NCC-1017, The Doomsday Machine)
Defiant(NCC-1764, Tholian Web)
Excalibur(NCC-1664, Ultimate Computer)
Exeter (NCC-1672, Omega Glory)
Hood (NCC-1703, Ultimate Computer)
Lexington(NCC-1709, Ultimate Computer)
Potempkin (NCC-1657, Ultimate Computer)
I questuion the following as "confirmed" constitution class ships, since they never actually appeared on screen. I can buy them being commonly accepted as Constitution class, but technically, we never saw visual confirmation:
Intrepid (NCC-1631, mentioned in the Immunity Syndrome, seen in Court Martial(?) in the remastered version...depends on your version of canon, I suppose)
Republic (NCC-1371, mentioned in Court Martial)
Farragut (mentioned in Obsession)
Yorktown (TVH, only see bridge shot and not confirmed if Enterprise-A is a rechristened Yorktown )
Abramsverse Enterprise (Unless I missed it, I never hear it called "Constitution class." For all we know, it could be the first of its kind, making it an Enterprise Class)