Don,
Now that David has done the hard part, by providing the correct and thorough answer, I will add my 2 cents.
If your locomotive is running well, if it was me, I would learn to live with a bad tender coupler that did not uncouple, but that is just me.
Also, if the locomotive is indeed running well, never take the boiler off if it can be avoided.
Additionally, if wanting to explore the condition of the wiring in the tender, if you notice a bad wire there, it is easier to replace that (with removing the tender shell), than on the locomotive.
Finally, please post pictures and explain the condition of the locomotive (runs well, runs poorly, does not run, etc.).
Tom