If thereīs doubt, then there is no doubt.
If you fly solely by NDB, then you have to have two operative NDBs onboard. So if one fails inflight, you canīt do NDB alone approach. If there are no other approaches available, then you divert, unless PiCīs emergency authority is invoked.
Regarding T-43 crash at Dubrovnik, Croatia, there was excellent article about it by Peter Garrisson, in Flying magazine, some ten years ago. It was called "Minor deviations" and I guess it made it to one of his "Aftermath" books. To sum it up: weather was marginal, approach was not stabilized (they crossed FAF at 210kt), sole ADF remained tuned to KLP (FAF) and not CV (LMM), procedure designer calculated too low MDA. And mountain had PK 1.