Now, the pending refit.... Are the issues with the core or the nacelles or both?
One fan-version of the "failure" that I worked on had the problem confined to the nacelles, what took so long refit wise was the construction standard warp nacelles in that size, essentially the delay was retooling the assembly line to make "standard" warp coils instead of "transwarp" coils. The revolutionary "warp-core" design stayed the same, and allowed the refit Excelsior with it's "Extra Beefy Standard Warp Drive" to actually set some of the promised speed and endurance records that Transwarp failed to achieve.
That's just one possible idea... another idea had them using "transwarp coils" reverse engineered from Borg technology and while it worked as promised, it attracted the attention of a Borg Sphere which was intercepted in the Tomed system by a joint Federation/Romulan taskforce. The Federation was forced to abandon that particular mode of Transwarp because it seemed to attract boogymen.
Seriously though, amazing work so far. Can't wait for the next installment.