Inbreeding can retain the 'stunner' genes from parents, and hence produce stunning fish. So that's a yes.
None-stunner fish can also, thanks to genetic recombination, produce stunning fish. Especially if they're not related (i.e. higher chance).
As for the appearance of genetic deformities in linebreeding, there's no hard and fast rule as to how long it takes before a species start showing signs of crappiness. Reason for this is because it's entirely stochastic, you can get deformities right off the bat, or you can inbreed a bunch of fish for 20 generations and see no deformities. Also depends on how you define 'deformity'. Technically all EBJDs are deformed...