As for the 19 year gap between C and D, do we know how long it could take to plan, design, and build a starship?
Why should that be a factor? Apparently, it took zero years to launch the E-B after the E-A was gone, or to launch the E-A once the E-nil was gone. One didn't have to design the E-B to succeed the E-A. One merely needed to name an available ship, any available ship,
Enterprise.
We don't have any clear indication that the E-B would have been the successor of the E-A in any practical sense. She may have been, much like the nuclear carrier
Enterprise was successor to the conventional carrier
Enterprise in the real world. But the two ships might also have been completely unrelated save for the name, just like the conventional carrier
Enterprise was unrelated to the preceding screw sloop
Enterprise in the real world.
From what we see and hear said on screen, it would appear that Kirk's first ship wasn't famous in TOS yet, but gained some fame before her destruction; that his second ship rode on the fame of the first one in the 23rd century; and that the E-B went on riding when launched, too, but was never considered noteworthy otherwise. The E-C was all but forgotten, until a bit of time travel rewrote history and her cryptic disappearance was changed to a heroic demise (and in the middle of the time reshuffling, our "war timeline" heroes thought that the name
Enterprise should be remembered by history - but perhaps only because theirs was going to be the first
Enterprise of note). By the time of the E-D, Starfleet heroes barely remember Kirk, let alone the history of his ship ("Naked Now", "Trials and Tribble-ations")...
Timo Saloniemi