CliffordK

There are some companies that seem to be doing excellent carbon fiber repair. But, it isn't cheap. Nonetheless, even if say steel is "repairable", in many cases all but the simplest of repairs exceed the value of a used repaired frame.

Duragrouch

That's not exactly the mechanism of a fatigue crack. If the seatpost was too small for the seat tube, the tube would flex too much when clamping and cause excessive stress there, exacerbating fatigue loading due to combined stresses. A loose "wobbling" seat post, if it banged on the end of wobble limits, could introduce additional shock loads, which may also be reversing, also possibly exacerbating fatigue loads. Lastly, one part sliding against another can introduce "fretting" wear, which can introduce a stress concentration or stress riser.